bluuebunny/crossref_metadata_2025 huggingface dataset:
Procedure for regulatory control structure selection with application to the FCC process
Authors: Hovd M. , Skogestad S.
Published: June 2004

Control structure (strategy) selection consists of the selection and pairing of manipulated and measured variables. This article outlines a procedure that uses such tools as the existence of right half plane (RHP) transmission zeros, the relative gain array, the performance relative gain array, and the closed‐loop disturbance gain. The regulatory control system for the fluid catalytic cracking process is used as an example. Several authors found the Kurihara control structure to be preferable to the conventional control structure. The reason is that RHP transmission zeros limit the achievable bandwidth for the conventional control structure. Two other control structures, however, have better controllability characteristics than both the conventional and the Kurihara control structures. The sensitivity of the measurement selection and variable pairing with respect to changes in the operating point and parametric uncertainty is examined, as well as the general objectives of the regulatory control level and its interaction with the higher levels in the control hierarchy.
Upper and lower limits of human skin electrical resistance in lontophoresis
Authors: Dinh Steven M. , Luo Ching‐Wang , Berner Bret
Published: June 2004

The time‐dependent electrical resistance of human cadaver skin was shown to be bound by an upper limit at equilibrium and a lower limit at steady state. The time to reach steady state from any initial condition by applying a current was much shorter than the recovery to equilibrium, due to different driving forces. The ratio of the equilibrium to the solution resistances increased with the salt concentration in free solution. This equilibrium relative resistance was sensitive to skin variability, primarily due to its dependence on porosity and pore geometry. A model was developed relating the equilibrium relative resistance to the structural properties of skin. The estimated pore radius of 25 × 10−10 m from the equilibrium resistance data is consistent with the average pore radius of human skin reported in the literature. In contrast, the steady‐state resistance could be several orders of magnitude lower than the equilibrium resistance and depended on current density and salt concentration but not on the history of prior exposures of a skin sample to a current. This difference can greatly reduce the power requirement for iontophoresis and can make this technology more practical. A linear relationship was found between the logarithm of the steady‐state relative resistance (skin/solution resistances) and the logarithm of the current density divided by the salt concentration. The slope of −0.65 was in excellent agreement with the theoretical slope of −2/3 that was derived for the convective transport of ions in a charged pore under a constant current.
Activated carbon adsorption and desorption of toluene in the aqueous phase
Authors: Chatzopoulos Dimitrios , Varma Arvind , Irvine Robert L.
Published: June 2004

The equilibrium and dynamics of toluene adsorption and desorption in singlecomponent aqueous solutions were investigated. Adsorption rates in a batch reactor under a variety of operating conditions were fitted successfully with the homogeneous surface diffusion model and a surface diffusion coefficient that increases exponentially with surface concentration. The dependence of the external mass‐transfer coefficient on the hydrodynamic conditions in the liquid phase was taken into account. Desorption studies in the aqueous phase and solvent regeneration of tolueneloaded activated carbon suggest that only a small fraction of toluene adsorbs irreversibly. Moreover, irreversible adsorption occurs only on virgin activated carbon, while adsorption on solvent‐regenerated activated carbon is fully reversible. After accounting for irreversible adsorption, the model successfully predicted toluene desorption rates in the aqueous phase under various operating conditions using toluene transport parameters determined from the adsorption studies.
Dynamic scaling of the cluster‐size distribul nucleation: Precoalescence stages
Authors: Shi Frank G. , Seinfeld John H.
Published: June 2004

The dynamic evolution of the cluster‐size distribution (CSD) in the precoalescence stages of nucleation and growth for sizes within and beyond the nucleation barrier layer (NBL) has been obtained. The existence of several universal kinetic laws of nucleation is predicted. In the precoalescence stages of nucleation, the CSD for sizes within and beyond the NBL obeys dynamic scaling relations. In a range of sizes beyond the NBL, the CSD exhibits a universal power law behavior, the exponent of which depends only on the interphase monomer transfer mechanism. Based on the results for the CSD, a general foundation is developed for nucleation kinetic measurements including the determination of the nucleation barrier and the inter‐facial monomer transport rate by measuring the CSD. A preliminary comparison with an experimental study confirms the predicted dynamic scaling and the power law behavior of the CSD in the early stages of nucleation and growth.
Modeling bubble and drop formation in flowing liquids in microgravity
Authors: Kim Ieehwan , Kamotani Yasuhiro , Ostrach Simon
Published: June 2004

A theoretical model for the process of bubble and drop formation in flowing liquids, applicable for both terrestrial and microgravity environments, has been developed by using a force balance. The contact angle variation at the nozzle due to the bubble motion and the added mass coefficient of the bubble moving through a pipe have been theoretically analyzed, considering bubble motions during its expansion and detachment stages. Predictions of bubble size of the model show satisfactory agreement with available experimental results in the case of normal gravity. The effects of the nondimensional variables on bubble and drop size are evaluated in microgravity conditions. In microgravity, the bubble is detached from the nozzle only by the liquid flow drag, and in the region of low liquid velocity the bubble size becomes much larger than that in normal gravity.
Breakthrough of lysozyme through an affinity membrane of cellulose‐cibacron blue
Authors: Liu Hsien‐Chang , Fried J. R.
Published: June 2004

The dynamic affinity adsorption oflysozyme through stacked flat‐sheet cellulose membranes with immobilized cibacron blue 3GA was studied and compared to three affinity‐membrane models: diffusion model to explore the importance of axial and radial diffusion; variation model to study the effects of pore‐size distribution or thickness variation; and stack model to investigate the effects of stacking flat‐sheet membranes. For the diffusion model, when Per > 0.1, radial diffusional resistance is significant, but when Pez < 25, axial dispersion must be considered. For the variation model, increasing pore‐size distribution or nonuniform membrane thickness greatly broadens the breakthrough curve. The stack model shows that the stacking of membranes significantly sharpens the breakthrough curves by averaging out the flow dispersion due to pore‐size distribution or thickness variation.
Nonlinear inferential control of pulp digesters
Authors: Lee Jay H. , Datta A. K.
Published: June 2004

Monitoring and control of batch pulp digesters, which convert wood chips to pulp by Kraft process are discussed. The Kappa number, which represents the extent of delignification, is the key controlled variable, which cannot be measured on‐line and must be estimated through secondary liquor measurements. Given a fixed batch time, the final Kappa number should be as close to the target Kappa number as possible, despite errors in the initial state estimates and input disturbances. To fulfill this objective, a state‐observer‐based model‐predictive controller is designed using a detailed nonlinear dynamic model of the digester. The extended Kalman filter (EKF) using on‐line measurements of various liquor characteristics is capable of recovering from significant errors in the initial state estimates. In addition, the EKF is shown to be robust to the errors in the covariance matrices and most model parameters, but quite sensitive to some model parameter errors. Coupled with the EKF, a finite‐horizon model predictive controller (MFC) based on successive linearization of the nonlinear pulping model, is found to work efficiently for controlling the Kappa number and batch time.
Tubular reactors for emulsion polymerization: I. Experimental investigation
Authors: Paquet D. A. , Ray W. H.
Published: June 2004

Experimental results using a pulsed tubular reactor are presented. The use of a pulsation source eliminates the reactor fouling and plugging issue that has occurred in previous investigations. Startup data and steady‐state profiles are presented. Comparisons in monomer conversion and particle‐size distribution for batch, CSTR and tubular reactor are made. The influence of pulsation on the particle‐size dis‐tribution is discussed. It is shown that high‐conversion latex with a narrow particle size distribution can be produced continuously in the pulsed tubular reactor.
Influence of increased gas density on hydrodynamics of bubble‐column reactors
Authors: Krishna Rajamani , De Swart Jeroen W. A. , Hennephof Daniëlle E. , Ellenberger Jürg , Hoefsloot Hubertus C. J.
Published: June 2004

A mechanistic background to the understanding of the hydrodynamics of high‐pressure bubble column reactors in both the homogeneous and heterogeneous flow regimes is discussed. An important parameter determining the stability of homogeneous bubbly flow in a bubble column is shown to be the Richardson‐Zaki exponent in the bubble swarm velocity relationship Vswarm = v∞(1‐ϵ)n‐1. Experimental data for the bubble swarm velocity were obtained in 0.05‐ and 0.1‐m‐dia. bubble columns with various gases (helium, air, argon, sulfur hexafluoride) using water as the liquid phase. Bubble swarm velocity data show that with increasing gas density the Richardson‐Zaki exponent value decreases; physically this means that increasing gas density reduces interaction between neighboring bubbles and, consequently, reduces chance of propagation of instabilities. This rationalizes the experimental observation that the influence of increased gas density ρG is to delay the transition from homogeneous bubbly flow to churn‐turbulent flow: increasing ρG increases the regime transition velocity. A stability analysis rationalizes the observations. The hydrodynamics of bubble columns in the churn‐turbulent regime is considered to be analogous to that of a bubbling gas‐solid fluidized bed, and the two‐phase theory of gas‐solid fluid beds is extended to describing bubble columns by identifying the “dilute” phase as the fast‐rising large bubbles and the “dense” phase as the liquid phase with entrained small bubbles. A simple coalescence rationalizes experimental large‐bubble holdup data.
Continuous‐Mixture kinetics of thermolytic extraction of coal in supercritical fluid
Authors: Wang Ming , Zhang Chunjie , Smith J. M. , McCoy Ben J.
Published: June 2004

Thermolysis of coal yields a complex mixture of many extract products whose molecular‐weight distribution (MWD) varies with time for continuous‐flow, semi‐batch experiments. A laboratory flow reactor with a differential fixed bed of coal particles contacted by supercritical tert‐butanol was used to provide dynamic MWD data by means of HPLC gel permeation chromatography of the extract. The experimental results, multipeaked, time‐dependent MWDs of extract molecules, are interpreted by a novel mathematical model based on continuous‐mixture kinetics for thermal cleavage of chemical bonds in the coal network. The parameters for the MWDs of extractable groups in the coal and the rate constants are determined from the experimental data. The effect of temperature on the kinetics of the extraction is explained in terms of one‐ and two‐fragment reactions of the extractable groups in the coal. At lower temperatures (613 and 633 K) single‐fragment reactions dominate, but at higher temperatures (653 and 673 K) more bonds in the coal are broken and the two‐fragment reactions become significant. Also, the detailed MWDs are related to conventional lumped data for the thermolytic extraction process. The results yield information about the structure of coal, as well as the kinetics and mechanism of supercritical thermolytic reactions.
Falling film evaporation of binary mixtures
Authors: Palen J. W. , Wang Qi , Chen J. C.
Published: June 2004

The interactive effects of heat and mass transfer in evaporation of binary mixtures flowing as falling films on vertical surfaces was investigated. Evaporative heat‐transfer coefficients were measured for aqueous mixtures of ethylene and propylene glycol, with boiling ranges up to 55°C. Tests were carried out at atmospheric pressure with heat fluxes ranging from 3,000 to 25,000 W/m2, and film Reynolds numbers ranging from 300 to 3,000. Results indicated that the heat‐transfer coefficient for mixtures depends weakly on wall superheat and film Reynolds number, but strongly depends on mixture composition. Analysis of the results indicates that mass‐transfer resistance in the liquid film causes significant elevation of the interface temperature, causing a reduction of the effective temperature driving force. A semiempirical model for correlation of the interactive heat‐and mass‐transfer phenomena is proposed.
Model‐predictive pH control using real‐time NARX approach
Authors: Pröll Thomas , Nazmul Karim M.
Published: June 2004

A new comprehensive real‐time identification/control methodology based on the concept of nonlinear autoregressive exogenous input (NARX) models and adaptive, nonlinear, model‐predictive control (ANMPC) is applied to a pH neutralization process. The existing NARX model theory has been extended by incorporating measured disturbances. NARX models have shown superior predictive characteristics in comparison to linear models. The proposed real‐time methodology uses a pointer vector being created during an initial identification and model structure selection procedure. Using this pointer vector, which allocates the chosen elements from the pool of all possible linear and nonlinear combinations, one needs no explicit information about the model structure for the closed‐loop control. The nonlinear programming problem encountered in ANMPC is solved by a gradient‐based modified Marquardt and finite difference methods. The design procedure and explicit algorithms are discussed for the multiinput/multioutput case. A pH wastewater neutralization process used illustrates and verifies the procedure by computer simulations and a real‐time laboratory‐scale experiments.
Estimating interfacial tension via relaxation of drop shapes and filament breakup
Authors: Tjahjadi Mahari , Ottino Julio M. , Stone Howard A.
Published: June 2004

Two methods are presented for calculating the interfacial tension between two immiscible Newtonian fluids. The procedures require only two measurements of the approximate interface shape during the low‐Reynold‐number interfacial‐tension‐driven transient motion of (i) the relaxation of a modestly elongated drop back to a spherical shape or (ii) the growth of capillary wave instabilities along the surface of a thread. The interfacial tension can be estimated by comparing the time differences between the two experimental measurements with tabulated and numerically generated results for the approximate shape evolution. The numerical results are generated using boundary integral method for similar model free‐boundary problems at low Reynolds numbers. The tabulated results are presented for drop to suspending fluid viscosity ratios 0.01 to 10.
Breakage and coalescence models for drops in turbulent dispersions
Authors: Tsouris C. , Tavlarides L. L.
Published: June 2004

A substantial effort has been made by numerous investigators to describe droplet breakage and coalescence in turbulent dispersions. An attempt is made here to improve these models based on existing frameworks and recent advances described in the literature. Two‐step mechanisms are considered for both the breakage and coalescence models. The drop breakage function is structured as the product of the drop‐eddy collision frequency and breakage efficiency which reflect the energetics of turbulent liquid‐liquid dispersions. The coalescence function retains the former structure of the product of drop‐drop collision frequency and coalescence efficiency. The coalescence efficiency model has been modified to account for the effects of film drainage for drops with partially mobile interfaces. These models overcome several inconsistencies observed in previous efforts and are applicable for dense dispersions (about ϕ[0.10–0.30]). For the daughter drops produced by breakage, a probability density is proposed based on the energy requirements for the formation of daughter drops.
On‐line molecular weight distribution estimation and control in batch polymerization
Authors: Ellis Mark F. , Taylor Tad W. , Jensen Klavs F.
Published: June 2004

A molecular weight distribution (MWD) estimator for batch methyl methacrylate solution polymerization is implemented experimentally for on‐line control and estimation of the MWD. The estimator is based on an extended Kalman filter and provides current estimates of the entire MWD, reaction temperature, monomer conversion, and initiator conversion. It uses a detailed polymerization model, on‐line monomer conversion measurements, temperature measurements, and periodic, time‐delayed measurements of the MWD from an on‐line size‐exclusion chromatograph. The estimator is shown to perform well with several on‐line MWD estimation experiments. Real‐time feedback control of the molecular weight is presented by utilizing the on‐line MWD information. Temperature, monomer‐addition, and simultaneous temperature and monomer‐addition control are investigated experimentally to achieve a specific constant weight‐average molecular weight. The on‐line feedback control is effective in rejecting realistic disturbances which deteriorate molecular weight control.
Synthesis of reactive mass‐exchange networks with general nonlinear equilibrium functions
Authors: Srinivas B. K. , El‐halwagi M. M.
Published: June 2004

The problem of synthesizing reactive mass‐exchange networks (REAMENs) is addressed for which the equilibrium relations governing the transfer of a certain species from rich streams to reactive mass‐separating agents are nonlinear (convex and/or nonconvex). Previously developed synthesis procedures for REAMENs are not applicable to such cases, since they were developed for cases of linear and/or convex equilibria. Two peculiar phenomena are associated with REAMENs with general nonlinear equilibria: the location of a mass‐exchangers may straddle the pinch point even for networks featuring the minimum cost of mass‐separating agents. To account for such intriguing characteristics, a systematic procedure is developed to synthesize cost‐effective REAMENs with general nonlinear equilibria. In addition, a robust solution procedure is devised to guarantee the identification of the global solution of the developed optimization programs. Applicability and usefulness of the proposed synthesis procedure are demonstrated via a case study on the desulfurization of gaseous wastes from a coal‐to‐methanol plant.
Dynamic modeling of product grade transitions for olefin polymerization processes
Authors: Debling J. A. , Han G. C. , Kuijpers F. , Verburg J. , Zacca J. , Ray W. H.
Published: June 2004

Most olefin polymerization processes produce a wide range of resin grades to suit market demand. Determining the transition policy between these grades is an important consideration that can affect process economics and safety. In this article, we compare several grade transition strategies and the relative performance of solution, slurry, bulk and gas‐phase processes for the production of polypropylene, linear low‐density polyethylene, high‐density polyethylene and ethylene‐propylene rubber. It is shown that the most important parameters that determine the grade transition performance of a process are reactor design, residence time, runtime per grade and residence time distribution of the polymer, gas and/or solution phases. Slow hydrogen dynamics severely retard grade changes involving increases in product molecular weight. The simulation results are obtained by using the dynamic simulation package “POLYRED” developed at the University of Wisconsin‐Madison.
Effect of pressure on the oxidative coupling of methane in the absence of catalyst
Authors: Chen Q. , Couwenberg P. M. , Marin G. B.
Published: June 2004

The oxidative coupling of methane was carried out in the absence of catalyst in a continuous flow setup at total pressures up to 1,000 kPa, temperatures from 950 to 1,230 K, and inlet molar ratios of CH4/O2 down to 2.5. At constant temperature and residence time, the conversions of methane and oxygen increase drastically with increasing pressure. At oxygen conversions higher than 80%, product selectivities are comparable at different pressures. The space‐time yield of the C2 products reaches a level comparable to that required for industrial operations from 400 kPa on. A radical‐reaction network consisting of 38 elementary reactions allows to describe the experimental data. To describe adequately the effect of total pressure, the pressure fall‐off behavior of the rate coefficients for the unimolecular reactions in the network has to be taken into account explicitly.

General features of the reaction mechanism do not depend on the total pressure. Methyl and hydrogen peroxy radicals are the most abundant radicals. The total pressure increase results in a drastic increase of the concentrations of the chain carriers, particularly the hydrogen peroxy radical. Higher pressure favor the oxidative route from ethane to ethylene compared to the pyrolytic route. Increasing the total pressure leads to an increase of the primary and a decrease of the consecutive CO formation relative to the coupling. The balance between these nonselective routes determines the effect of the total pressure on the integral selectivity to C2 products at different conversions. The major contribution to the integral CO selectivity comes from the oxidation of methyl radicals.
Solubilization in nonionic reverse micelles in carbon dioxide
Authors: McFann Gregory J. , Johnston Keith P. , Howdle Steven M.
Published: June 2004

Phase behavior data and FTIR spectra along with solubilities and solvatochromic shifts of ionic indicator dyes are reported for binary, ternary and quaternary systems composed of pentaethylene glycol n‐octyl ether (C8E5), CO2, water, and n‐pentanol as a function of temperature, pressure and composition. With the addition of n‐pentanol as a cosurfactant, the water to surfactant ratio, W0, reaches values as high as 12. As water is added further, a surfactant‐rich phase precipitates due to micellemicelle interactions. Ionic dyes are soluble at levels of 0.03 mM, far in excess of the negligible solubility in pure CO2.
Reduction of ion‐exchange equilibria data using an error in variables approach
Authors: Vamos Richard J. , Haas Charles N.
Published: June 2004

An error in variables method (EVM) was used to reduce binary ion‐exchange equilibria data. Ion‐exchange equilibria data were collected via batch equilibrium experiments. Three binary ion‐exchange systems were studied: the Na+1‐Cd+2, Na+1‐Cu+2, and Cu+2‐Cd+2 systems with a strong acid synthetic ion‐exchange resin. The Wilson model and a three‐suffix, two‐parameter Margules equation were used to model the dependency of the resin phase activity coefficients with resin‐phase composition. The EVM was used to determine the best‐fit estimates of the Wilson and Margules equation binary resin‐phase activity coefficient parameters. Statistical analysis of the data fits indicates that although both equations were able to adequately model the resin phase nonidealities, the Wilson model provided the superior data fits based on the minimized objective functions.
Use of relay‐feedback test for automatic tuning of multivariable systems
Authors: Shen Shih‐Haur , Yu Cheng‐Ching
Published: June 2004

The Åström‐Hägglund autotuner is extended to multivariable systems. The proposed multivariable automatic tuning performs the identification‐design procedure in a sequential manner. Its advantages are discussed, as well as the process characteristics resulted from sequential design. One important feature is that the sequential design often leads to underdamped process characteristics. Given these characteristics, potential problems in the Ziegler‐Nichols tuning are discussed and a modification is made. The sequential identification and the modified Ziegler‐Nichols controller design method form the basic structure for the multivariable autotuner. The properties of convergence, the implications of tuning sequence, and the indications of undesirable pairing for the autotuner are explored. Consequently, a multivariable autotuning procedure is proposed. Two nonlinear distillation examples and a 3 × 3 linear system are used to illustrate the effectiveness of the autotuner. Simulation results show that good performance can be obtained with minimal engineering effort and the autotuner works well for very difficult processes in a very simple way.
Centrifugation assay of IgE‐mediated cell adhesion to antigen‐coated gels
Authors: Chu Lily , Tempelman Linda A. , Miller Cynthia , Hammer Daniel A.
Published: June 2004

The adhesion of biological cells to substrates is often mediated by binding between cellular receptors and substrate‐bound ligand. In this work, we used a centrifugation assay to measure the adhesion of rat basophilic leukemia (RBL) cells coated with immunoglobulin E (IgE) to substrates coated with the ligand dinitrophenol (DNP). Increasing force, decreasing DNP substrate density, and decreasing cell surface IgE density all led to decreasing adhesion. Experiments performed at low IgE cell surface densities, in which few tethers from between cell and substrate suggest individual tethers have a binding strength of 2 to 4 microdyne, in agreement with previous measurements of the force to uproot receptors from the plasma membrane. We use this system to show how subpopulations expressing different numbers of cell surface receptors may be separated by exploiting their differential adhesiveness to substrates.
Two methods of selecting smoothing splines applied to fermentation process data
Authors: Thornhill Nina F. , Manela Mauro , Campbell John A. , Stone Karl M.
Published: June 2004

Two methods for generating smoothing splines are compared and applied to data from a fed‐batch fermentation process. One method chose both the degree of the spline and its parameters by minimizing the generalized cross validation (GCV) function using a genetic algorithm (GA). The other method adjusted the smoothing spline to a specified chi‐square goodness‐of‐fit, requiring prior knowledge of the measurement variability. The GCV/GA method led to excellent results with all the fermentation data records. The goodness‐of‐fit method gave a family of spline fits; splines with a low percentage fit extracted trends from the data, while for general use a 50% fit appeared satisfactory. The goodness‐of‐fit method executed more quickly than the GCV/GA method, but the GCV/GA method was more generally applicable as it chose both the degree of the spline and the amount of smoothing automatically.
Process monitoring and diagnosis by multiblock PLS methods
Authors: MacGregor John F. , Jaeckle Christiane , Kiparissides Costas , Koutoudi M.
Published: June 2004

Schemes for monitoring the operating performance of large continuous processes using multivariate statistical projection methods such as principal component analysis (PCA) and projection to latent structures (PLS) are extended to situations where the processes can be naturally blocked into subsections. The multiblock projection methods allow one to establish monitoring charts for the individual process subsections as well as for the entire process. When a special event or fault occurs in a subsection of the process, these multiblock methods can generally detect the event earlier and reveal the subsection within which the event has occurred. More detailed diagnostic methods based on interrogating the underlying PCA/PLS models are also developed. These methods show those process variables which are the main contributors to any deviations that have occurred, thereby allowing one to diagnose the cause of the event more easily. These ideas are demonstrated using detailed simulation studies on a multisection tubular reactor for the production of low‐density polyethylene.
Diagnosis of multiple simultaneous fault via hierarchical artificial neural networks
Authors: Watanabe Kajiro , Hirota Seiichi , Hou Liya , Himmelblau D. M.
Published: June 2004

We discuss a new type of macroarchitecture of neural networks called a HANN and how to train it for fault diagnosis given appropriate data patterns. The HANN divides a large number of patterns into many smaller subsets so the classification can be carried out more efficiently via an artificial neural network. One of its advantages is that multiple faults can be detected in new data even if the network is trained with data representing single faults. The use of a HANN is illustrated in fault diagnosis of a chemical reactor.
Stability in optimal design: Synthesis of complex reactor networks
Authors: Kokossis A. C. , Floudas C. A.
Published: June 2004

A systematic methodology applicable to the optimal design of stable process systems is presented. It is based on the formulation of a parametric problem that provides bounds on the optimal stable solution and an iterative algorithmic approach that attains convergence of the bounds in a finite number of iterations. The bounds on the optimal stable solution are based on analytical expressions of bounds on the eigenvalues of the Jacobian matrix using the concept of the measure of the matrix. When extended to the synthesis problem of reactor networks, the approach is able to couple the optimization problem with stability issues even in cases where the number of reactors is large and the reaction mechanism is described by a general complex reaction scheme. Furthermore, since at the synthesis level the reactor network represents an exhaustive superposition of the existing structural and operational alternatives, the approach fully exploits these alternatives and coordinates a weighted optimal search that improves the objective and accommodates a stable reactor network. This approach is not restricted to the synthesis of reactor networks and can be applied to the design of total process flowsheets.
Formulations for three‐phase flash calculations
Authors: Chien Henry Hung‐Yeh
Published: June 2004

A study of flash equations associated with multicomponent systems that can form three phases at a given temperature and pressure indicated that different equations are needed at different boundaries of the solution space. This problem dictates that one‐ and two‐phase trial solutions be obtained first on the boundaries prior to a search in the three‐phase interior region. Two new formulations of the flash equations have been developed to avoid this problem. Solutions at boundaries as well as those in the three‐phase region can now be obtained with a single search procedure. The new formulations are made using functions that are monotonic in the selected coordinates. These properties facilitate the use of nested, single‐variable search methods. Interpolating with second‐order functions, when applicable, nested search methods are often faster than two‐dimensional simultaneous search methods. Tests of these formulations have demonstrated the advantages of these characteristics.
Modeling of combined discrete/continuous processes
Authors: Barton P. I. , Pantelides C. C.
Published: June 2004

The dynamic behavior of processing systems exhibits both continuous and significant discrete aspects. Process simulation is therefore a combined discrete/continuous simulation problem. In addition, there is a critical need for a declarative process modeling environment to encompass the entire range of processing system operation, from purely continuous to batch. These issues are addressed by this article.

A new formal mathematical description of the combined discrete/continuous simulation problem is introduced to enhance the understanding of the fundamental discrete changes required to model processing systems. The modeling task is decomposed into two distinct activities: modeling fundamental physical behavior, and modeling the external actions imposed on this physical system. Both require significant discrete components. Important contributions include a powerful representation for discontinuities in physical behavior, and the first detailed consideration of how complex sequences of control actions may be modeled in a general manner.
Constraint handing and stability properties of model‐predictive control
Authors: de Oliveira Nuno M. C. , Biegler Lorenz T.
Published: June 2004

Effects of hard constraints in the stability of model‐perdictive control (MPC) are reviewed. Assuming a fixed active set, the optimal solution can be expressed in a general state‐feedback closed form, which corresponds to a piecewise linear controller for the linear model case. Changes in the original unconstrained solution by the active constraints and other effects related to the loss of degrees of freedom are depicted in this analysis. In addition to modifications in the unconstrained feedback gain, we show that the presence of active output constraints can introduce extra feedback terms in the predictive controller. This can lead to instability of the constrained closed‐loop system with certain active sets, independent of the choice of tuning parameters. To cope with these problems and extend the constraint handling capabilities of MPC, we introduce the consideration of soft constraints. We compare the use of the l2‐(quadratic), l1‐(exact), and l∞‐norm penalty formulations. The analysis reveals a strong similarity between the control laws, which allows a direct extrapolation of the unconstrained tuning guidelines to the constrained case. In particular, the exact penalty treatment has identical stability characteristics to the correspondent unconstrained case and therefore seems well suited for general soft constraint handling, even with nonlinear models. These extensions are included in the previously developed Newton control framework, allowing the use of the approach within a consistent framework for both linear and nonlinear process models, increasing the scope of applications of the method. Process examples illustrate the capabilities of the proposed approaches.
Self‐diffusion coefficients of sol‐gel intermediates
Authors: Ng Li Voon , McCormick Alon V.
Published: June 2004

Sol‐gel techniques have shown great potential for producing ceramic materials of designed composition and properties. A typical sol‐gel process involves the hydrolysis and polymerization of one or more metal alkoxides in an alcohol solution in a batch reactor to form a homogeneous gel, which is then further treated to provide the type of ceramic desired (such as aerogel, glass, crystal, and coating). Several authors have suggested that diffusion effects may influence the development of the gel microstructure. We investigate what governs the diffusivity of the precursors using the pulsed‐gradient‐spin‐echo NMR technique. This initial report focuses on the early stages of the batch reaction.

The system chosen is the acid catalyzed hydrolysis and condensation of tetraethyl orthosilicated in ethanol. 1H PGSE NMR and 29Si NMR show that the diffusivities of the silicate precursors are strong functions both of the molecular weight and of hydrogen bonding between the polar functional groups and the solvent. Moreover, since hydrolysis increases and polymerization reduces the concentration of these polar groups, the mobility of the reactive oligomers changes in a complex manner. Combining our results from PGSE NMR and gas chromatography, we estimate the extent of hydrogen bonding effect on the mobility of various oligomers, providing a model to predict diffusivity trends throughout the batch reaction.
Vapor‐liquid equilibrium of polymer solutions using a cubic equation of state
Authors: Orbey Nese , Sandler Stanley I.
Published: June 2004

A recently proposed mixing rule has been combined with a cubic equation of state to correlate the vapor‐liquid equilibria of various polymer + solvent and solvent + long chain hydrocarbon mixtures. We find that the two‐parameter version of this model can correlate the solvent partial pressure in concentrated polymer solutions with high accuracy over a range of temperatures and pressures with temperature‐independent parameters. For the solvent + long chain hydrocarbon systems, for which accurate vapor‐phase concentrations are also important, the model is best used as a three‐parameter correlation. Advantages of this approach are that it extends the application of cubic equations of state to polymer + solvent systems in a simple fashion and that generally only temperature‐independent parameters are required which allow for accurate interpolation and extrapolation of available data.
Structure of the stretching field in chaotic cavity flows
Authors: Liu M. , Peskin R. L. , Muzzio F. J. , Leong C. W.
Published: June 2004

Stretching of material elements in time‐periodic cavity flows is investigated numerically. The spatial structure of the stretching field is determined not only by nonchaotic islands and by unstable manifolds of hyperbolic periodic points, but also by singularities of the flow field at the cavity corners. For the short time scales interesting to most mixing applications, regions of very high stretching (good local mixing) are determined by unstable manifolds that pass close to the corners of the cavity. Low stretching (poor local mixing) regions are usually found both inside and near islands. In some cases, however, the unstable manifolds wrap themselves around the islands, preventing the formation of segregated low stretching subregions within the chaotic region.
Pressure‐dependent gas heat transport in a spherical pore
Authors: Wolf Jeffrey R. , Strieder William C.
Published: June 2004

A mean free path gas kinetic theory is used to model the conductive heat transport of a gas within a void volume enclosed in a Fourier solid. A variational upper bound principle is derived for a void of arbitrary shape and applied to obtain a rigorous upper bound equation for the void gas conductivity in a spherical void. The variational void gas conductivity equation is exact in both the large and small Knudsen number (Kn) limits and provides a means to determine the accuracy of the reciprocal additivity interpolation formula as applied to thermal conductivity rather than diffusive mass transfer (maximum error 6% at Kn = 0.5 and α = 1). Temperature jump will occur even at atmospheric pressures and higher for sufficiently small thermal accommodation coefficients (α < 0.1). Experimental void gas heat conductivities vs. pressure data for H2, He, Ne, N2, CO2, and F12 in a polyurethane foam are compared with theoretical mean free path void gas conductivity vs. inverse Knudsen number curves drawn for various α. Estimates of the thermal accommodation coefficients for the gas‐ polyurethane surface exhibit a maximum with increasing molecular mass of the gas molecules, which qualitatively agrees with the predictions of Baule's classical theory. Results also point to a rather sharp shift of the S curve to higher pressures with decreasing thermal accommodation.
Modeling of coal pyrolsis kinetics
Authors: Liliedahl Truls , Sjöström Krister
Published: June 2004

Four different coals were instantaneously introduced into a laboratory‐scale pyrolysis chamber at various temperatures, and their develatilization profiles were analyzed with respect to kinetics. The experimental setup consisted of a gas chromatograph (GC) with a pyrolysis unit, an empty column, and a detector. Detector signals were digitalized and stored for further handling. Normally, 1,024 values were sampled at 250 Hz. For deconvolving the true instantaneous devolatilization from the observed detector response, the residence time distribution was compensated for.

An empirical model approach is developed to correlate the total devolatilization rates when instantaneously introducing a particle in a constant temperature environment. The model is also extended to pyrolysis application.
A new reactor for purely homogeneous kinetic studies of endothermic reactions
Authors: Safvi S. A. , Mountziaris T. J.
Published: June 2004

A new counterflow jet reactor has been designed to study the purely homogeneous kinetics of endothermic reactions. The reactor consists of two vertical, coaxial, counterflowing, laminar jets and radial‐flow exit region. It can be used to generate a reaction zone near the stagnation point and away from walls, thus eliminating the possibility of surface reactions. One jet is heated and contains only a suitable carrier gas such as hydrogen and nitrogen, while the other is unheated and contains the compound(s) under study diluted in the same carrier gas. A 2‐D model of the process has been used to simulate the thermal decomposition of tertiary‐butyl‐arsine, a precursor for metal‐organic chemical vapor deposition of GaAs films. Performance diagrams based on Reynolds and Demköhler numbers were constructed to identify optimal operating conditions and to demonstrate the feasibility of the technique. This reactor appears to be an attractive choice for studies of the purely homogeneous kinetics of endothermic reactions at pressures close to atmospheric.
Solute‐induced effects on the structure and thermodynamics of infinitely dilute mixtures
Authors: Chialvo Ariel A. , Cummings Peter T.
Published: June 2004

A general molecular‐based formalism developed rigorously establishes microscopic bases of the supercritical solubility enhancement in terms of well‐defined molecular correlation function integrals by unambiguously splitting the mixture's properties into short‐(finite) and long‐range (diverging) contributions. Consequently, the short‐range nature of the solute's and solvent's residual chemical potentials is proved and the change of the solvent's local environment around an infinitely dilute solute and its finite contribution to solute mechanical partial molar properties are interpreted in terms of the short‐range solute‐solvent and solvent‐solvent direct correlation function integrals. The solute‐induced effect on the system's microstructure and thermodynamics approaches zero as the mixture approaches ideality or the solute becomes an ideal gas particle. At the solvent's critical conditions, the solute‐induced effect on the solute's partial molar properties shows no compressibility‐driven singularity, though along the critical isotherm it can exhibit a finite extremum or a change of curvature near critical density, depending on the type and strength of solute‐solvent interactions. The utility of the proposed solvation formalism is illustrated using statistical mechanical integral equation calculations for three simple models of infinitely dilute near‐critical mixtures: pyrene‐CO2, diterbutyl nitroxide‐ethane, and Ne‐Xe.
Viscous film flow in the stagnation region of the jet impinging on planar surface
Authors: Yeckel Andrew , Strong Lisa , Middleman Stanley
Published: June 2004

A capillary jet of liquid impinges on a planar surface that is normally oriented to the axis of the jet. The surface is initially covered with a thin uniform film of a viscous liquid. The impact and radial spreading of the liquid from the jet cause the underlying viscous film to be removed from the surface. An approximate analysis predicts the thinning rate of the film in the stagnation region of the jet. It uses the shear stress and pressure distribution of the classical Homann flow as boundary conditions for an analytical solution of the Reynolds lubrication equations in this underlying viscous film. A more exact analysis modifies the Homann flow to account for the mobility of the liquid film beneath the spreading jet and sheds light on the limitations of the analytical lubrication analysis. Data presented are in excellent agreement with the theory, subject only to the choice of a value for the hydrodynamic constant that appears in the Homann analysis.
Analysis of steric hindrance effects on adsorption kinetics and equilibria
Authors: Jin Xuezhi , Talbot Julian , Wang N.‐H. Linda
Published: June 2004

Most existing adsorption models do not properly consider steric hindrance effects of preadsorbed solutes. As a consequence, the models often fail to represent the adsorption kinetics and equilibria accurately. In this work, we extend the random sequential adsorption concept for irreversible adsorption to analyze reversible adsorption on a continuous surface and a random site surface. Based on simulation results of these processes, general kinetic equations for one‐component adsorption are developed. The equations are used to correlate chromatography frontal curves of lysozyme and isotherm data of ethane adsorption on activated carbon and ethylene adsorption on a molecular sieve. The significance of the equations, as compared with the Langmuir equation, lies not only in their ability to correlate the experimental data more accurately, but in the physical significance of the adsorption parameters such as the maximum adsorption capacity obtained from the correlation. Our study shows that steric hindrance effects alone result in nonlinear Scatchard and Hill plots with negative cooperativity.
Modeling the role of bacteria in leaching of low‐grade ores
Authors: Batarseh Kareem I. , Stiller Alfred H.
Published: June 2004

A robustious structural model is developed to describe the role of bacteria in the leaching process of low‐grade ores under conditions controlled by intraparticle diffusion. The main impetus behind developing this model is to provide an insight into such systems, together with a suitable framework for interpreting experimental data. The model is derived in detail with respect to reaction chemistry and the role of bacteria in catalyzing these reactions, specifically the synergism of chemistry, physics and biology in determining the overall behavior of the system. The model is used to simulate the atmospheric oxidation of iron disulfide contained in porous solids in the presence of Thiobacillus ferrooxidans (T. ferrooxidans). The experimental data are predicted well by the model, which demonstrates its applicability and supports the view that the rate of intraparticle diffusion is the controlling mechanism for this system.
Analysis of capillary‐driven viscous flows during the sintering of ceramic powders
Authors: Martínez‐Herrera Jorge I. , Derby Jeffrey J.
Published: June 2004

Time‐dependent viscous flows driven by capillarity act to minimize the surface area of a fluid bounded in a plane geometry with initial gradients in surface curvature. These free‐surface flows are solved by a finite‐element model applied to describe the viscous sintering of two‐dimensional ceramic particles. The numerical model is validated by comparison to the analytical solution obtained by Hopper (1990) for the coalescence of two infinite cylinders of equal cross section and is applied to several other geometries pertinent to the study of particle sintering for which analytical results are not available. Details of the flow fields and morphological evolution lend insight to the physical behavior of these systems and provide a basis for the more complete understanding of viscous sintering phenomena.