bluuebunny/crossref_metadata_2025 huggingface dataset:
Design parameters estimation for scale‐up of high‐pressure bubble columns
Authors: Wilkinson Peter M. , Spek Arie P. , van Dierendonck Laurent L.
Published: June 2004

Experiments were carried out in bubble columns for a number of liquids at pressures between 0.1 and 2.0 MPa for two column sizes. Based on the experimental results as well as extensive literature data, the extent of the effect column dimensions have on gas holdup were determined, both at low and high pressures (which is of importance to scale‐up). It was also demonstrated that none of the published empirical gas holdup equations incorporate the influence of gas density accurately. Therefore, a new improved gas hold‐up equation is developed that incorporates the influence of gas and liquid properties with an average error of approximately 10%. Finally, it is also discussed to what extent theinfluence of pressure on other important design parameters such as the interfacial area, the liquid volumetric mass transfer coefficient, and gas and liquid mixing, can be estimated on the basis of empirical equations.
The robust global stabilization of a stirred tank reactor
Authors: Adebekun K.
Published: June 2004

In this paper, a program of analysis is carried out for control of a nonlinear stirred tank reactor. By examining the uniqueness and stability properties of the reactor, the zero dynamics are analyzed. Subsequently, a model reference adaptive controller (MRAC) which explicitly takes into account the nonlinearities of the system is designed; the MRAC successfully compensates for parametric uncertainty. It is shown that even when almost all reactor parameters are unknown, global convergence over the entire phase space is achieved.
A theoretically correct mixing rule for cubic equations of state
Authors: Wong David Shan Hill , Sandler Stanley I.
Published: June 2004

A new mixing rule developed for cubic equations of state equates the excess Helmholtz free energy at infinite pressure from an equation of state to that from an activity coefficient model. Use of the Helmholtz free energy insures that the second virial coefficient calculated from the equation of state has a quadratic composition dependence, as required by statistical mechanics. Consequently, this mixing rule produces the correct low‐ and high‐density limits without being density‐dependent.

As a test, the mixing rule is used for ternary mixtures of cyclohexane + benzene + water, ethanol + benzene + water and carbon dioxide + n‐propane + water, and all the constituent binaries. The new mixing rule and a simple cubic equation of state can be used for the accurate correlation of vapor‐liquid and liquid‐liquid equilibria for binary mixtures. Using the parameters obtained from binary systems, the phase behavior of ternary mixtures can be predicted. Also, unlike previous empirical mixing rules, this theoretically based mixing rule is equally applicable and accurate for simple mixtures containing hydrocarbons and inorganic gases and mixtures containing polar, aromatic and associating species over a wide range of pressures. This mixing rule makes it possible to use a single equation of state model with equal accuracy for mixtures usually described by equations of state and for those traditionally described by activity coefficient models. It is the correct bridge between these two classes of models.
Modeling fixed‐bed coal gasifiers
Authors: Hobbs Michael L. , Radulovic Predrag T. , Smoot L. Douglas
Published: June 2004

A one‐dimensional model of countercurrent fixed‐bed coal gasification has been developed, and results have been compared to experimental data from commercial‐scale gasifiers. The steady‐state model considers separate gas and solid temperatures, axially variable solid and gas flow rates, variable bed void fraction, coal drying, devolatilization based on chemical functional group composition, oxidation and gasification of char, and partial equilibrium in the gas phase. Generalized treatment of gas‐phase chemistry and accounting for variable bed void fraction were necessary to predict realistic axial temperature and pressure profiles in an atmospheric fixed‐bed gasifier. Model evaluation includes sensitivity of axial temperature profiles to model options, model parameters and operational parameters. Model predictions agree reasonably well with experimental temperature and pressure profile data for gasification of eight coal types ranging from lignite to bituminous. The relative importance of char oxidation resistances to bulk film diffusion, ash diffusion, and chemical reaction is identified.
Morphology of polymers precipitated from a supercritical solvent
Authors: Lele Ashish K. , Shine Annette D.
Published: June 2004

The precipitation of polymers via the rapid expansion of a supercritical chlorodifluoromethane solution to ambient conditions across a fine diameter capillary has been studied experimentally. The morphology of the polymers precipitated—polycaprolactone, poly(methyl methacrylate) and a styrene/methyl methacrylate block copolymer—is influenced strongly by conditions of the expansion process. Conditions of high temperature, high polymer concentration, low pressure or low capillary L/D ratio enhance the formation of high aspect ratio fibers, while opposite conditions favor the formation of spherical particles of micron size. Each of the conditions favoring fiber formation favors precipitation farther upstream in the expansion process. Based on one‐dimensional compressible flow calculations using a virial equation of state for pure solvent, it is proposed that fiber formation occurs when a polymer‐rich phase is rejected from solution in the entry region to the capillary. The location of precipitation is shown to be crucial in determining the characteristic time scale for the density reduction process, which may be as small as 10−7 s.
Thermal convection, morphological stability and the dendritic growth of crystals
Authors: Koo Kee‐Kahb , Ananth Ramagopal , Gill William N.
Published: June 2004

New data on the dendritic growth and microstructure of ice crystals in quiescent pure water are reported for small subcoolings of 0.035 K <ΔT<1.000 K, where thermal or natural convection is prevalent. Accurate and systematic measurements of the growth velocity VG and the tip radii of the edge and basal planes R1 and R2 were made as functions of time.

The central point of this work is that with the harmonic mean of the tip radii Rm as the lengthscale, the intensity of natural convection can be correlated accurately by using the thermal convection analogy, Gr = Re2. On this basis, natural convection has a crucial effect on dendritic growth of ice at ΔT<0.35 K, the region of subcooling in which the tip of the dendrite splits consistently.

The experiments show that the morphological stability parameter C* is independent of subcooling and equals 0.075, when the lengthscale is Rm. With the observed values of R1 and R2, the aspect ratio is 28, and the growth velocity for small ΔT is significantly higher than that predicted by the conduction theory of Horvay and Cahn (1961). Thus, the effect of convection on the growth of ice crystals is more important as the subcooling decreases.

Moving boundary solutions of the three‐dimensional Navier‐Stokes and energy equations for the dendritic growth of an elliptical paraboloid were obtained here with the Stokes flow approximation. Experimental observations of the quantities, VG, R1, and R2, agree well with predictions of this theory when Gr=Rem2 is based on Rm. In contrast, if convection is neglected in the theory, it does not agree with the experiments and the difference increases significantly as the subcooling is decreased.
Dynamic nonisothermal transport in hygroscopic porous media: Moisture diffusion in wood
Authors: Avramidis Stavros , Englezos Peter , Papathanasiou Thanasis
Published: June 2004

A model that predicts heat and moisture transfer in the hygroscopic range of a complex porous material such as wood, was evaluated with unsteady‐state nonisothermal diffusion experimental data. Water chemical potential gradient was taken as the driving force for diffusion, and the derivation of the temperature‐gradient phenomenological coefficient in the mass balance equation was based on the principles of nonequilibrium thermodynamics. The results reveal an excellent prediction of the specimen's average moisture content during desorption in the hygroscopic range. Moreover, a very good agreement was also shown between the specimen's center temperature and the model predictions. The model revealed the existence of a thermal‐diffusion phenomenon during the initial stages of the desorption process. This phenomenon was not predicted by Fick's equation for diffusion.
Identification and cross‐directional control of coating processes
Authors: Braatz Richard D. , Tyler Matthew L. , Morari Manfred , Pranckh Ferdinand R. , Sartor Luigi
Published: June 2004

Of special industrial interest is the cross‐directional control of coating processes, where the cross direction refers to the direction perpendicular to the substrate movement. The objective of the controller is to maintain a uniform coating under unmeasured process disturbances. Assumptions that are relevant to coating processes found in industry are used to develop a model for control design. This model is used to derive a model predictive controller to maintain flat profiles of coating across the substrate by varying the liquid flows along the cross direction. Actuator constraints, measurement noise, model uncertainty, and the plant condition number are investigated to determine which of these limit the achievable closed‐loop performance. From knowledge of how these limitations affect the performance we can make some recommendations on how to modify the plant design to improve the coating uniformity. The theory developed throughout the article is rigorously verified through experiments on a pilot plant. The controller rejects disturbances with two sampling times. The proposed controller can reduce the variance in coating thickness by as much as 80% compared to what is possible by manual control or simple control schemes.
Fluid flow in compressible porous media: II: Dynamic behavior
Authors: Jönsson K. Ann‐Sofi , Jönsson Bengt T. L.
Published: June 2004

This article deals with the dynamic behavior during filtration and wet pressing of compressible porous media. An equation is developed which permits the derivation of the time dependency of porosity and flow at varying locations in a medium subjected to hydraulic pressure and/or mechanical load. Furthermore, a numerical solution that allows the equation to be solved rapidly and with high accuracy on a common PC is presented. The computer program developed in conjunction with this work has been used to illustrate the dynamics of fluid flow in some industrially important applications, such as wet pressing of paper pulp and flow in chromatography columns. The program has also been used to calculate the flow and concentration variations during mechanical impulse loading (that is, a press pulse).
High‐temperature oxidation resistance of SiO2‐coated polyimide composite
Authors: Neogi Sudarsan , Gulino Daniel A.
Published: June 2004

Silicon dioxide films (500–5,000 Å‐thick) were deposited on silicon wafers and polyimide composite surfaces by plasma‐enhanced chemical vapor deposition. The chemical reaction takes place on the surface and depends on substrate conditions and process parameters. In the present study, SiO2 films were deposited from tetraethylorthosilicate (TEOS) in the 275–400°C temperature range under the influence of a radio frequency discharge (13.56 MHz). The properties of the deposited films, such as refractive index, density, stoichiometry, and adhesion, were obtained. The deposition characteristics of the films were studied as a function of process parameters including temperature, pressure, and RF power level. Finally these TEOS‐SiO2 films were found to be effective in the protection of polyimide‐based composites for higher‐temperature (350–400°C) applications.
Use of dielectric loss to determine blend time for organic polymer solutions
Authors: Shervin Carl R. , Studeny Joseph E. , Romaszewski Raymond A.
Published: June 2004

When scaling up mixing applications, it is often important to measure blend time. In our application, we need to measure blend time in nonaqueous solutions of organic polymers in pilot‐ and plant‐scale steel tanks. Techniques employing visual observations can be used in small‐scale glass or Plexiglas tanks, but these methods are not applicable to large‐scale operations. Another standard technique is to measure conductivity of a tracer material, which is useful particularly for aqueous systems, but does not provide an adequate response for our system.

A new technique that we established involves the measurement of the dielectric loss of a tracer material added to an acrylic polymer solution in mineral oil. It is demonstrated that the addition of N,N‐dimethylformamide (DMF) results in mixing curves that give blend times equivalent to those measured by visual methods. Our preliminary studies were performed in a 2‐L vessel over a temperature range of 30–120°C. The magnitude of the dielectric loss measurement is shown to depend on the scanning frequency of the measurement and the temperature of the polymer solution. The variation over a small temperature range is slight and will not interfere with the measurements.
Imaging velocity profiles: Flow through an abrupt contraction and expansion
Authors: Xia Yang , Callaghan Paul T. , Jeffrey Kenneth R.
Published: June 2004

Dynamic nuclear magnetic resonance (NMR) microscopy has been used to study the laminar flow of water through cylindrical tubes in the vicinity of both an abrupt expansion and an abrupt contraction. Radial profiles of the velocity have been obtained, at a spatial resolution on the order of 20 μm, for the velocity components in the axial and transverse directions, both at the junction and at different axial displacements on either side of the junction. The data have been compared with solutions of the Navier‐Stokes equations obtained by finite difference numerical methods using a Macintosh personal computer. Excellent agreement is apparent without the need for any adjustable parameters, the numerical solutions depending only on the Reynolds number determined from inlet flow conditions. This study demonstrates the usefulness of dynamic NMR microscopy for the study of the flow of Newtonian fluids in complex geometries. The method can also be applied to the flow of non‐Newtonian fluids.
Transport phenomena during freezing of isolated hepatocytes
Authors: Toner Mehmet , Tompkins Ronald G. , Cravalho Ernest G. , Yarmush Martin L.
Published: June 2004

The cellular response of isolated rat hepatocytes to freezing stress under various chemical and physical conditions was investigated using cryomicroscopic observations and a theoretical transport model. Cryomicroscopy was used to determine the cellular dehydration during freezing at different cooling rates. Using these results, water permeability parameters were also obtained. In addition, intracellular ice formation (IIF) parameters were obtained from rapid freezing experiments at a cooling rate of 400°C/min to decouple the effects of dehydration from IIF. The water permeability and IIF parameters were then used in a transport model to predict the probability of IIF under a wide range of chemical and physical conditions observed during cell freezing. In order to verify the model predictions, experiments were performed under similar conditions and compared to the theoretical predictions. The results suggest that slowly cooled cells (< 80°C/min) dehydrate excessively and escape IIF. On the other hand rapidly cooled cells (> 120°C/min) contain enough undercooled cellular water which undergoes IIF between −5 and −12°C.
Two‐dimensional finite element analysis of microwave heating
Authors: Ayappa K. G. , Davis H. T. , Davis E. A. , Gordon J.
Published: June 2004

Transient temperature profiles for long rods of lossy dielectric materials with thermally‐dependent dielectric properties exposed to uniform plane waves are obtained. Maxwell's equation and the heat equation are simultaneously solved using the finite element method to predict the power absorbed and the resulting temperature rise in samples of square and circular cross‐section. Following the method introduced recently, we derive an exact radiation boundary condition which is independent of the rod cross‐section. For a cylindrical sample, the boundary condition is imposed on the cylinder itself. For a square rod, the boundary condition is imposed on a cylinder containing the rod. The temperature dependence of dielectric properties and sample dimensions appreciably influence heating patterns. For square samples, the edges focus radiation, causing preferential heating at the edges. This effect is pronounced for larger samples. In addition, the incident wave polarization influences the heating of the rod. For waves where the electric field is polarized along the long axis of the sample (TMz polarization) the power absorbed is higher than when the electric field is perpendicular to the axis (TEz polarization). A case involving runaway heating is also investigated.
Wrong‐way behavior of packed‐bed reactors: Influence of reactant adsorption on support
Authors: Il'in Andrew , Luss Dan
Published: June 2004

Wrong‐way behavior refers to a large transient temperature increase caused by a sudden reduction in the feed temperature or increase in the feed rate to a packed‐bed reactor operating at an intermediate or high level of conversion. This dynamic temperature rise may be affected by reactant adsorption on the inert catalyst support. The wrong‐way behavior usually leads to formation of a downstream‐moving temperature front. In such cases, reactant adsorption tends to moderate and decrease the maximal transient temperature of these fronts. However, when the wrong‐way behavior generates an upstream‐moving temperature front, reactant adsorption may substantially increase the temperature rise over that attained in its absence and ignite the reactor. Reactant adsorption may also lead to surprising dynamic effects upon changes in feed velocity.
Sedimentation of monodisperse and bidisperse hard‐sphere colloidal suspensions
Authors: Al‐Naafa M. A. , Selim M. Sami
Published: June 2004

The effect of concentration on the sedimentation rate of uncharged rigid spheres was investigated. Three submicron sizes of silica spheres were prepared according to the method of Stöber (1968). The particles were sterically stabilized by chemisorption of stearyl alcohol at their surface by the method developed by van Helden (1981). The sterically stabilized silica particles dispersed in cyclohexane are known to behave as hard spheres. Monodisperse gravity sedimentation experiments were carried out for the various particle species over a wide concentration range of ϕ = 0.003 to ϕ = 0.37, where ϕ is the particle volume fraction in cyclohexane. The data for dilute suspensions (ϕ < 0.03) were found to be well described by Batchelor's equation: U/U0 = 1–6.55ϕ, where U is the sedimentation velocity and U0 is the Stokes velocity of a sphere in isolation. The data over the entire concentration range (0<ϕ<0.37) were found to be well described by the equation: U/U0 = (1–ϕ)6.55.

Bidisperse sedimentation experiments were also carried out, and the dilute data were found to be well represented by Batchelor's 1982 theory for polydisperse suspensions. The high concentration data were analyzed in terms of a model that does not distinguish between interactions of like and unlike particles.
Supercritical phase fischer‐tropsch synthesis: Catalyst pore‐size effect
Authors: Fan Li , Yokota Kohshiroh , Fujimoto Kaoru
Published: June 2004

The Fischer‐Tropsch synthesis reaction was conducted in a supercritical fluid medium using a fixed‐bed reactor. Tailor‐made catalyst supports which had sharp pore diameter distributions were prepared by the pH swing method. The relationships between the catalyst pore structure and the catalytic activity or the product distribution were studied. The influence of the catalyst pore size on the mass transfer of reactants and products was also characterized. The catalyst pore size affected not only catalytic activity but also product selectivity. The diffusion of reactants inside the catalyst pellets in the supercritical fluid media was simulated and the effects of catalyst pore size and catalyst particle size on catalytic performances were consistent with simulation results.
Mixed‐gas adsorption dynamics of high‐concentration components in a particulate bed
Authors: Frey Douglas D.
Published: June 2004

The study of LeVan et al. (1988) concerning the dynamics of mixed‐gas adsorption of components having high concentrations is extended to the case of multicomponent isotherms determined from either ideal or nonideal adsorbed solution theory. A discussion is given of the effect of isotherm properties on the dynamics of isothermal, mixed‐gas adsorption when the adsorbate concentrations in the feed gas are large enough so that the interstitial fluid velocity varies significantly along the adsorption front.
Design considerations for tubular reactors with highly exothermic reactions
Authors: Shinnar Reuel , Doyle Francis J. , Budman Hector M. , Morari Manfred
Published: June 2004

Practical guidelines are required for the design and operation of complicated catalytic packed‐bed reactors. Primary among design considerations is the avoidance of operating regions of high parametric sensitivity, in which small changes in operating conditions can lead to thermal runaway in the reactor. Existing criteria for predicting these regions rely on complex mathematical formulations for differential sensitivity between input and output variables. The present work centers on the development of practical design criteria for avoiding reactor instability and temperature sensitivity in multitubular packed‐bed reactors. A set of simple guidelines is proposed for the sizing of reactors and proper selection of operating conditions. The implications of these open‐loop sensitivities for a controlled reactor are investigated. It is shown through simulation studies that violation of the proposed criteria leads to control problems and difficulty in operating at the design point.
Comparative study of flow schemes for a simulated countercurrent adsorption separation process
Authors: Ching C. B. , Chu K. H. , Hidajat K. , Uddin M. S.
Published: June 2004

Continuous separation of three carbohydrate mixtures [fructose‐dextran (M. W. ≈ 9,400), raffinose‐dextran (M. W. ≈ 6,000), and fructose‐raffinose] has been carried out using a simulated countercurrent process with silica gel as the sorbent and deionized water as the eluent. Experiments were conducted using the commercial four‐section arrangement and a modified three‐section flow scheme which does not recycle part of the eluent for reuse. Data for the four‐section flow scheme reflect a much higher product concentration in the raffinate and a substantially lower rate of fresh eluent consumption than for the three‐section scheme. It is concluded that when both the extract and raffinate products are desired, the four‐section scheme should be used for the separation of a mixture with a small separation factor which usually requires a large quantity of eluent to achieve an efficient separation. The behavior of the simulated countercurrent process was simulated theoretically by representing the process in terms of an equivalent true countercurrent system. A good fit to the experimental data was obtained when the process was configured in the four‐section mode. However, the same theoretical approach failed to provide a good representation of the behavior of the three‐section scheme in one respect: predicted concentrations of the raffinate product were about two times higher than experimental values. The discrepancy may be attributed to the periodic switching of fluid inlet and outlet points in the operation of the three‐section simulated countercurrent unit.
Predictive correlation for binary diffusion coefficients in dense carbon dioxide
Authors: Funazukuri Toshitaka , Ishiwata Yoshihisa , Wakao Noriaki
Published: June 2004

Binary diffusion coefficients of some organic compounds in carbon dioxide at 313.2 K and 16.0/25.0 MPa were measured by using the Taylor‐Aris tracer response technique. We propose a new correlation of Schmidt numbers as a function of solvent molar volumes for predicting binary diffusion coefficients in dence CO2 and self‐diffusion coefficients of dense CO2. The correlation was also found to be valid for predicting self‐diffusion coefficients of dense CH4 at Fv/A* < 40 or v2/(ṽ2)0 > 1.62.
Dense, vertical gas‐solid flow in a pipe
Authors: Miller Aubrey , Gidaspow Dimitri
Published: June 2004

The hydrodynamics of gas‐solid flow, usually referred to as circulating fluidizedbed flow, was studied in a 7.5‐cm clear acrylic riser with 75‐μm FCC catalyst particles. Data were obtained for three central sections as a function of gas and solids flow rates. Fluxes were measured by means of an extraction probe. Particle concentrations were measured with an X‐ray densitometer. In agreement with previous investigators, these data showed the flow to be in the core‐annular regime, with a dilute rising core and a dense descending annular region. However, unlike the previous studies conducted worldwide, the data obtained in this investigation allowed us to determine the viscosity of the suspension. The viscosity was a linear function of the volume fraction of solids. It extrapolates to the high bubbling‐bed viscosities.
Bed expansion of liquid‐solid inverse fluidization
Authors: Karamanev D. G. , Nikolov L. N.
Published: June 2004

The bed expansion characteristics of two‐phase inverse fluidization were studied. Twelve different spheres with diameters from 1.31 to 7.24 mm and densities between 75 and 930 kg/m3 were fluidized with water. The experimental lne‐lnU curves were parallel to those predicted by the model of Richardson and Zaki and therefore the exponents n were similar. However, Ui, the liquid velocity at ε = 1 differed from that predicted from the standard drag curve for Ret > 130. This can be explained by the fact that the drag curve of a freely rising light sphere differs from that of a falling particle. The values of Ui calculated using this modified drag curve were in good agreement with the experimental results. The difference between experimental and calculated from the Ergun equation minimal fluidization velocities is explained by the difference in mechanical inertia of the light and heavy particles.
Discrete‐time nonlinear controller synthesis by input/output linearization
Authors: Soroush Masoud , Kravaris Costas
Published: June 2004

This work concerns the synthesis of discrete‐time nonlinear controllers for nonlinear processes that make the closed‐loop system linear in an input/output sense. The synthesis of state feedback controllers is studied first, followed by the synthesis of dynamic output feedback controllers. Both problems are solved within the globally linearizing control (GLC) framework. Precise theoretical connections between the derived controllers and model algorithmic control (MAC) are established. The theory is illustrated by a chemical reactor example.
An analysis of the statistical model adsorption isotherm
Authors: Kaminsky R. D. , Monson P. A.
Published: June 2004

The statistical model adsorption isotherm developed by Ruthven and coworkers and used in the prediction and correlation of adsorption in microporous materials especially zeolites is critically examined from the perspective of statistical mechanics. This is done by applying the method to a class of molecular models for which the statistical thermodynamics may be solved analytically without approximation. The models considered are finite length single component and binary one‐dimensional systems in which the molecules interact via square well potentials. These are among the simplest realistic yet analytically solvable molecular models of adsorption in porous materials. Our analysis clarifies the theoretical status of the Ruthven approach as well as revealing some of the weaknesses in it.
Thickening theories—an analysis
Authors: Fitch Bryant
Published: June 2004

There is as yet no theoretically sound and empirically proven procedure that is fully reliable for the design of thickeners. From a logical standpoint, a sound procedure is one correctly deduced from valid premises or assumptions. As noted in Fitch's earlier work(979), there is no reason to believe that existing mathematical models are based on uniformly valid assumptions. There is also much reason to believe that they are not. Therefore existing mathematical models or “theories” of thickening are analyzed for mathematical consistency, scientific credibility, and relevance to chemical engineering. Domains in which the models are useful approximations of reality and thus are relevant to chemical engineering practice are identified. Emendations or alternatives are suggested to bring the models more nearly into conformity with empirical results. Design procedures are suggested that, although involving assumptions that cannot be uniformly valid, would seem more reasonable than existing ones.
Drag relationships for liquid droplets settling in a continuous liquid
Authors: Greene G. A. , Irvine T. F. , Gyves T. , Smith T.
Published: June 2004

Experiments are reported in which the drag of single liquid droplets settling in a tall column of another lighter immiscible liquid are measured. The experimental data for the eight pairs of liquids that were tested covered a range of droplet Reynolds numbers from 10−3 to 104. Two regimes of droplet settling were encountered. In the first regime, the droplets remained spherical, and the drag agreed very well with established solid sphere drag models. In the second regime, the droplets became deformed and oscillated; the drag was found to depart suddenly from predictions of spherical models and to increase with increasing Reynolds number. Empirical models for the point of departure from spherical drag and the coefficient of drag in the unstable regime are derived.
An approach to fault diagnosis of chemical processes via neural networks
Authors: Fan J. Y. , Nikolaou M. , White R. E.
Published: June 2004

This article presents an approach to fault diagnosis of chemical processes at steadystate operation by using artificial neural networks. The conventional back‐propagation network is enhanced by adding a number of functional units to the input layer. This technique considerably extends a network's capability for representing complex nonlinear relations and makes it possible to simultaneously diagnose multiple faults and their corresponding levels in a chemical process. A simulation study of a heptane‐to‐toluene process at steady‐state operation shows successful results for the proposed approach.
Cosolvent interactions in supercritical fluid solutions
Authors: Ekart Michael P. , Bennett Karen L. , Ekart Susan M. , Gurdial Gurdev S. , Liotta Charles L. , Eckert Charles A.
Published: June 2004

The addition of cosolvents to supercritical fluid (SCF) solvents can have large effects on solubilities, giving engineers the ability to tailor loadings and selectivities of solutes for difficult separations. It is necessary to have a better understanding of the special intermolecular interactions that occur in SCF solutions to predict the effects of cosolvents. We use a SCF chromatographic technique to acquire a database of cosolvent effects for a variety of cosolvents and solutes; examination of the cosolvent effects shows evidence of hydrogen bonding, charge transfer complex formation, and dipole‐dipole coupling between solute and cosolvent molecules. SCF solvents, carbon dioxide, ethane, and fluoroform, are compared, and then the use of the chromatograph to measure solubilities is discussed.
Nonlinear transport processes in disordered media
Authors: Sahimi Muhammad
Published: June 2004

Nonlinear transport processes in disordered systems such as porous media and heterogeneous solids are studied, which are represented by two‐ or three‐dimensional networks of interconnected bonds, by a Bethe network (a branching network with no closed loops) of a given coordination number, or by a continuum in which circular or spherical inclusions have been inserted at random. The bonds represent the pores of the pore space, or the conducting and insulating regions of a disordered solid, to which we assign effective properties (radii or conductances) selected at random from a probability density function. Three types of nonlinear transport processes are considered. (1) The relation between the current q and the potential gradient v is of power‐law type (as in, for example, flow of power‐law fluids or the electric current in doped polycrystalline semiconductors). (2) The relation between q and v is piecewise linear, characterized by a threshold (as in flow of Bingham fluids or in mechanical or dielectric breakdown of composite solids). (3) A large v is imposed on the system, so that a linear transport theory is not valid. The behavioral study of the effective transport and topological properties of the system, such as the permeability, conductivity, diffusivity, and the shape of the samplespanning cluster of conducting paths shows that in all cases the concepts of percolation theory play a prominent role, even if the system is well connected and percolation may seem not to play any role. For most cases, new effective‐medium approximations (EMAs) are derived for estimating effective transport properties. Compared to the case of linear transport, new EMAs are considerably more accurate in predicting the scaling properties of the transport coefficients near a critical point such as the percolation threshold. For a power‐law transport process, an exact solution is also derived for the Bethe networks. Using the concepts of percolation theory, scaling laws relating the effective properties to various regimes of transport and to topological properties of the system are also given. A relation between the volumetric flow rate of a power‐law fluid in porous media and the macroscopic pressure drop is derived, which contains no adjustable parameter and is valid at any porosity. To test the accuracy of our analytical predictions, Monte Carlo simulations are carried out for several cases. In most cases, good agreement is found between the simulation results and predictions. The extension of the results to other types of nonlinearities is also discussed.
Physically representative network models of transport in porous media
Authors: Bryant Steven L. , Mellor David W. , Cade Christopher A.
Published: June 2004

We calculate permeabilities for a class of granular porous media derived from a real, disordered packing of equal spheres. The entire structure, including pore space, of these media is completely specified by the radii and spatial locations of the constituent grains. When geometric nearest neighbor grains are grouped together, the structure may be subdivided into pore bodies and pore throats in a natural and unambiguous way. From this subdivision we can establish a network of flow paths whose geometry and topology are completely specified, so that permeability and other transport coefficients can be calculated directly and without any adjustable parameters. The calculations focus on processes that form porous media, rather than on specific examples of such media. Hence, the approach is essentially predictive, rather than correlative. No additional measurements (such as capillary pressure data or pore system data from thin sections) are required, and correlations between permeability and other properties are not used. Predicted permeabilities match measurements on sandstone samples similar to the model perous media studied here over a wide range of porosity. Geometrical attributes of the network representation of the pore space of the model media are found to be spatially correlated. This departure from randomness significantly affects permeability. The agreement between predictions and measurements suggests that spatial correlation is inherent in granular porous media and that uncorrelated network models are therefore unlikely to be physically representative of such media.
Robust design of binary countercurrent adsorption separation processes
Authors: Storti Giuseppe , Mazzotti Marco , Morbidelli Massimo , Carrà Sergio
Published: June 2004

The separation of a binary mixture, using a third component having intermediate adsorptivity as desorbent, in a four section countercurrent adsorption separation unit is considered. A procedure for the optimal and robust design of the unit is developed in the frame of Equilibrium Theory, using a model where the adsorption equilibria are described through the constant selectivity stoichiometric model, while mass‐transfer resistances and axial mixing are neglected.

By requiring that the unit achieves complete separation, it is possible to identify a set of implicit constraints on the operating parameters, that is, the flow rate ratios in the four sections of the unit. From these constraints explicit bounds on the operating parameters are obtained, thus yielding a region in the operating parameters space, which can be drawn a priori in terms of the adsorption equilibrium constants and the feed composition.

This result provides a very convenient tool to determine both optimal and robust operating conditions. The latter issue is addressed by first analyzing the various possible sources of disturbances, as well as their effect on the separation performance. Next, the criteria for the robust design of the unit are discussed. Finally, these theoretical findings are compared with a set of experimental results obtained in a six port simulated moving bed adsorption separation unit operated in the vapor phase.
Impact of catalyst acid/metal balance in hydroisomerization of normal paraffins
Authors: Degnan T. F. , Kennedy C. R.
Published: June 2004

The hydroisomerization of n‐heptane has been used as a model to study the acidmetal balance requirements for Pt/zeolite catalysts. Experiments were carried out using physical mixtures of zeolite beta and Pt/Al2O3 particles, in which the level of the metal‐containing component was varied to change the acid‐metal balance. Selectivity, in terms of cracking vs. isomerization, varies significantly with the metal loading up to a point beyond which the degree of paraffin isomerization is independent of metal loading. These results are interpreted in terms of a simple dualsite model based on a classical theory of polyfunctional catalysis, first advanced by Weisz (1962). The model is also used to show how an imbalance in hydrogenation and acid functions can even alter the apparent reaction network of the observable chemical species in the system.
Characterization of flow‐through electrode processes by AC impedance
Authors: Yuh C. Y. , Selman J. R.
Published: June 2004

AC‐impedance method has been used successfully in characterizing a packed‐bed flow‐through electrode system. A macrohomogeneous model was developed to simulate the effect of structural, physical and flow parameters. The relative importance of kinetics and mass transfer can be inferred from the AC‐impedance analysis. Kinetic information about copper deposition in supported cupric sulfate solution has been obtained successfully using this technique.
Theoretical study of recycling in preparative chromatography
Authors: Seidel‐Morgenstern Andreas , Guiochon Georges
Published: June 2004

Using the equilibrium‐dispersive model of chromatography, we have investigated recycling as a means to improve the recovery yields and possibly the production rates of chromatographic separations. When compared with the optimal values obtained for the conventional single‐cycle regime, our results prove the potential for recycling in preparative chromatography. The analysis performed for the separation of two binary mixtures demonstrates that recycling not only improves the recovery yield but can increase the production rate under certain conditions. Different possibilities of collecting sufficiently separated fractions during recycling are compared. The mathematical model developed can be used effectively for designing optimal recycling strategies in preparative liquid chromatography.
Nonlinear product property control in industrial gas‐phase polyethylene reactors
Authors: McAuley K. B. , Macgregor J. F.
Published: June 2004

A nonlinear model‐based scheme is developed for product property control in industrial gas‐phase polyethylene reactors. The controller regulates instantaneous melt index and density, and provides servocontrol during grade changeovers. Hydrogen and butene feed rates are manipulated to force the product properties onto desired trajectories. During grade changeovers, these trajectories are determined from off‐line dynamic optimization studies. Optimal open‐loop policies for reactor temperature, bleed stream flow, catalyst feed rate, and bed level are implemented as part of the changeover strategy.

The nonlinear feedback controller design is based on global input/output linearization methods. Disturbances are estimated, and plant/model mismatch is removed using an extended Kalman filter. Simulations on a complex mechanistic model of the process reveal that the nonlinear controller performs well for both regulatory and servocontrol. An analogous linear IMC controller is inadequate for disturbance rejection at different operating conditions and for control during grade changeovers. The simplicity of the nonlinear control algorithm makes it an interesting candidate for industrial application. This single controller can be used to control the properties of many grades of polyethylene made in the reactor and to accomplish near optimal changeovers between these grades.
Activity coefficients in dilute aqueous solutions from free energy simulations
Authors: Lazaridis Themis , Paulaitis Michael E.
Published: June 2004

The free energy perturbation method with Monte Carlo simulations has been used to calculate relative solvation free energies and ratios of activity coefficients of organic solutes at infinite‐dilution in water at 25°C. Systems studied include hydrocarbons, chlorinated hydrocarbons, and alcohols. Analysis of the free energy simulations reveals that a major limitation of current methodologies is our inability to determine a priori the electron density distribution of a molecule in a condensed, polarizing medium. Moreover, currently employed simulation lengths may be inadequate for convergence of the averages in some aqueous systems. These problems do not, however, appear to be insurmountable. In contrast to group contribution methods, free energy simulations are still rather primitive to be applied in a routine fashion for the quantitative prediction of thermodynamic properties, but dramatic improvements in the technique are still to be made.
Computation of tricritical points in ternary systems
Authors: Kohse Bruce F. , Heidemann Robert A.
Published: June 2004

At tricritical points, four equations in the thermodynamic variables must be satisfied. Methods of evaluating the functions and solving these four equations have recently been developed (Michelsen and Heidemann, 1988). A modified approach to the evaluation of these functions and a new solution procedure for the calculation of tricritical points in equation of state models are presented here. The new solution procedure is evaluated in comparison to previous ones, and its application is made using the Peng‐Robinson and Soave‐Redlich‐Kwong equations of state. The new procedure proved capable of locating tricritical points in a variety of systems with complex multiphase behavior. These included ternary systems with single tricritical points, systems with an ordinary critical point occurring at the same composition as a tricritical point and systems capable of four‐phase equilibria with two tricritical points. Comparisons are made with experimental tricritical data for eight systems. The calculations prove to be highly sensitive to pure component and binary parameters in the equation of state models. Features of phase diagrams for tricritical mixtures are discussed as well as the predictions of tricritical points in several homologous series of ternary systems.
Mechanistic model of slug flow in near‐horizontal pipes
Authors: Andreussi P. , Minervini A. , Paglianti A.
Published: June 2004

A mechanistic model of slug flow based on a set of mass and momentum conservation equations and a number of empirical closure relations is presented. It is then shown that a simplified version of the model, which physically corresponds to the flow of long liquid slugs(long slug model, LSM), can be immediately derived from the general model. The LSM is much simpler than the general model; at the same time, the predictions obtained in the two cases are very similar, when the comparison is limited to the computation of the pressure gradient and the mean liquid holdup. The LSM successfully correlates a large set of experimental measurements relative to three pipe diameters(18, 50 and 90 mm), four inclinations (0°, ±3°, 0.3°), and two pipe lengths(17 and 34 m).
A simple model of cross‐flow filtration based on particle adhesion
Authors: Stamatakis K. , Tien Chi
Published: June 2004

A simple model presented for cross‐flow filtration of liquid suspensions was formulated on the premise that among the particles convected to the filter medium surface in cross‐flow filtration, only a fraction of them become deposited. A criterion based on the interplay of the geometry of the cake‐suspension interface and various forces acting on a particle as it moves toward the interface was established and an expression of the adhesion probability of impacting particles developed. The model was then formulated by incorporating the adhesion probability information together with conventional cake filtration theories. Through sample calculations, the model was found to display behavior consistent with the observed phenomena of cross‐flow filtration and capable of representing experimental results.
Pyrolysis of scrap tires and conversion of chars to activated carbon
Authors: Merchant Akbar A. , Petrich Mark A.
Published: June 2004

The primary objective of this work was to demonstrate the conversion of scrap tires to activated carbon. We have been successful in this endeavor, producing carbons with surface areas greater than 500 m2/g and significant micropore volumes. Tire shreddings were pyrolyzed in batch reactors, and the pyrolysis chars activated by reaction with superheated steam. Solid products of pyrolysis and activation were studied with nitrogen adsorption techniques. We find that the porosity development during steam activation of tire pyrolysis char is similar to that reported for various other chars. A maximum in micropore volume is observed as a function of conversion, but the total surface area increases monotonically with conversion. We suggest that the activation process consists of micropore formation, followed by pore enlargement. The process conditions used in this study are a good starting point from which to optimize a process to convert tires to activated carbon.
Particle dispersion in a binary solid‐liquid fluidized bed
Authors: Asif Mohammad , Petersen James N.
Published: June 2004

The mixing and segregation behavior of liquid fluidized beds containing two different‐sized particle species are described mathematically. This model is based on a mass balance, rather than a volume balance, which has been used by most previous researchers. The model also includes the effects of density gradients that are present in the bed. Particle dispersion coefficients in a binary‐solid fluidized bed are evaluated using this model. Further, a correlation that allows the calculation of these particle dispersion coefficients is presented, which describes not only dispersion coefficients calculated using the above model, but also those presented by other researchers. It is applicable over a broad range of variables of practical interest and is based on relevant dimensionless parameters to account for the influence of important particle and fluid properties on the particle dispersion characteristics.