bluuebunny/crossref_metadata_2025_split huggingface dataset:
Enhanced automatic tuning procedure for process control of PI/PID controllers
Authors: Tan K. K. , Lee T. H. , Wang Q. G. 
Published: June 2004

AbstractAn enhanced automatic tuning procedure developed for process control of PI and PID controllers addresses several potential problems present in current standard autotuners. The proposed enhanced autotuner uses a novel technique based on relay feedback to estimate the process frequency response at two specified phase lags on the Nyquist curve automatically. An iterative procedure then uses these two points to obtain a transfer‐function model of the process. Based on this model and a controller‐selection scheme, an appropriate controller (PI or PID) is applied to the process automatically. The controller is tuned so that the Nyquist curve of the compensated system is appropriately shaped to satisfy a combined gain and phase‐margin type of specification. The effectiveness of this enhanced autotuner is demonstrated both in simulations and in real‐time experiments for level control of a coupled‐tanks system.
Design of robust nonsquare constrained model‐predictive control
Authors: Sarimveis Haralambos , Genceli Hasmet , Nikolaou Michael 
Published: June 2004

AbstractA model‐predictive control (MPC) design methodology for processes with more manipulated inputs than outputs is developed. Essential features of the proposed approach are the following: the on‐line optimization minimizes an objective function based on the l2 norm; an end‐condition equation is utilized; model uncertainty is considered as upper and lower bounds on the pulse‐response‐model coefficients; hard constraints on the input and move‐size variables and soft constraints on the output variables are posed. A major difference between square and nonsquare MPC is that in the former the end‐condition can be used directly, while in the latter a nonlinear programming problem needs to be solved during the design phase to select values for the input move suppression coefficients. This technique is illustrated through a number of simulations and application to a real industrial process.
Gas holdup in bubble column reactors operating in the churn‐turbulent flow regime
Authors: Krishna Rajamani , Ellenberger Jürg 
Published: June 2004

AbstractA comprehensive experimental study of gas holdup in bubble columns of varying diameters, fitted with different distributor types, using several liquids is presented. Air was used as the gas phase. Experiments to test the influence of gas density were also carried out with He, Ar, and SF6. A generalization of the two‐phase model for gas–solid fluidized beds was used to interpret the experimental data where the “dilute” phase is identified with the “large” bubble population and the “dense” phase with the liquid phase where the “small” bubble population is entrained. Gas holdups in dilute and dense phases were determined from dynamic gas disengagement experiments.In the churn‐turbulent regime of operation, voidage of the gas in the dense phase was independent of the superficial gas velocity. Reilly et al.'s correlations for the gas holdup and superficial gas velocity at the regime transition point estimate the gas voidage of the dense phase and the superficial gas velocity well through this phase. Corresponding correlations of Wilkinson et al. significantly underpredict dense‐phase parameters. The experiment showed that the dilute phase or large bubble holdup in bubble columns, operating at superficial gas velocities > 0.1 m/s, is independent of liquid properties, how the gas is distributed and the density of the gas phase. But it is affected significantly by the column diameter. Relying on hydrodynamic analogies with a gas–solid–fluid bed, a simple correlation was developed that is considerably more accurate than the Wilkinson correlation that significantly overpredicts large bubble holdup.
Angular temperature variations in a wall‐cooled packed‐bed reactor
Authors: Schouten E. P. S. , Westerterp K. R. 
Published: June 2004

AbstractThe significance of the statistical character of wall‐cooled packed‐bed reactors was studied by measuring angular temperature variations, which result from the random nature of the packing. These are neglected in present‐day mathematical models designed to describe the reactor behavior. The amplitude of these variations was determined experimentally as a function of the mass flux and the position in the tube, under conditions of heat transfer as well as of reaction. Angular variations are significant whenever radial temperature gradients are large.Different methods used to average observed variations were surveyed. For the system presented, the angular variations have only a limited influence on the reaction rates if the angular averaged temperature is used. Thus, a two‐dimensional deterministic continuum model can be used, notwithstanding the statistical character of the packed bed.
Model for the reaction‐rate‐limited dissolution of solids with Etch‐rate heterogeneities
Authors: Robertson Eric A. , Fogler H. Scott 
Published: June 2004

AbstractThe coalescence of isotropic etch pits observed in the dissolution of semiconductor substrates is studied using a discrete model for the evolution of the surface under reaction‐rate‐limited conditions. The model discretizes the solid into cubic elements and repetitively applies dissolution rules to the individual elements. The rate of mass removal is based on the number and arrangement of the element's exposed faces and the specified reaction‐rate parameters. Detailed knowledge of the surface normal is not required. The model shows that even at moderate etch pit densities, the effects of the coalescence do not significantly alter the trends observed for noncoalescing etch pits.
Steady and unsteady computation of impeller‐stirred reactors
Authors: Harvey Albert D. , Rogers Stuart E. 
Published: June 2004

AbstractA general computational approach is presented for numerical modeling of viscous flow in baffled, impeller‐stirred‐tank reactors. A multiblock, body‐fitted grid structure facilitates modeling of various impeller and baffle designs, and a new procedure offers averaged velocity data from a complex 3‐D CFD dataset. Impellers are modeled precisely, eliminating the need for inputting experimental velocity data for boundary conditions. The method can be used quickly to obtain extremely detailed flow computations at a fraction of the cost of computing unsteady moving grid solutions. A steady‐state computational approach that neglects the relative motion between impeller and baffles yields numerical results comparably accurate to full unsteady computations for laminar flow at a fraction of the time and expense. The approximate steady‐state method is used to predict power requirements of a Rushton turbine in laminar flow.An unsteady, moving grid technique provides time‐accurate solutions for the flow inside an impeller‐stirred reactor with side‐wall baffles. These computed results are compared with those using the approximate steady‐state method and with experimental measurements. The unsteady, moving grid method uses two different initial conditions: one starting from rest and the other starting from an approximate steady‐state solution obtained at the starting position of the impeller relative to the baffles. For unsteady simulations of laminar flow in stirred vessels, the final operating condition can be achieved much more efficiently if the solution obtained from the steady‐state procedure is used as an approximate initial condition.
Boiling on a straight pin fin
Authors: Lin W. W. , Lee D. J. 
Published: June 2004

AbstractIn this study, Multimode boiling on a straight pin is theoretically investigated. Axial steady‐state temperature distributions along the fin are numerically evaluated, as well as their linear stability characteristics. When film and transition boiling coexist on the fin surface, or only the transition boiling covers the entire fin, the operation remains stable only if the fin length is less than some critical value. When transition and nucleate boiling coexist on a fin, or the fin is in the three‐mode boiling (film + transition + nucleate boiling), the entry of nucleate boiling at the fin tip stabilizes the boiling process. This study on base heat flow and fin efficiency with the stability criteria also suggests a new fin design methodology.
Robust design of countercurrent adsorption separation: 3. Nonstoichiometric systems
Authors: Mazzotti Marco , Storti Giuseppe , Morbidelli Massimo 
Published: June 2004

AbstractThe separation of a multicomponent mixture in a countercurrent adsorptive separation unit is analyzed. A procedure for the optimal and robust design of the operating conditions of the unit is developed in the frame of equilibrium theory, where the adsorption equilibria are described through the constant selectivity nonstoichiometric Langmuir model, while mass‐transfer resistances and axial dispersion are neglected. This allows for extending the results previously obtained for systems characterized by either the linear or stoichiometric Langmuir isotherm to cases where the adsorbates exhibit rather different loading capacities. Analogies and differences among these three equilibrium models with respect to the criteria for the choice of operating conditions of countercurrent separation units are discussed. A comparison between model predictions and experimental data assesses the reliability and accuracy of the theoretical results achieved in this work.
Identification of faulty sensors using principal component analysis
Authors: Dunia Ricardo , Qin S. Joe , Edgar Thomas F. , McAvoy Thomas J. 
Published: June 2004

AbstractEven though there has been a recent interest in the use of principal component analysis (PCA) for sensor fault detection and identification, few identification schemes for faulty sensors have considered the possibility of an abnormal operating condition of the plant. This article presents the use of PCA for sensor fault identification via reconstruction. The principal component model captures measurement correlations and reconstructs each variable by using iterative substitution and optimization. The transient behavior of a number of sensor faults in various types of residuals is analyzed. A sensor validity index (SVI) is proposed to determine the status of each sensor. On‐line implementation of the SVI is examined for different types of sensor faults. The way the index is filtered represents an important tuning parameter for sensor fault identification. An example using boiler process data demonstrates attractive features of the SVI.
On‐line optimization of batch processes in the presence of measurable disturbances
Authors: Rahman Shamsur , Palanki Srinivas 
Published: June 2004

AbstractFeedforwar/state feedback laws are developed for on‐line optimization of batch reactors in the presence of measurable disturbances. The degrees of singularity with respect to manipulated input and disturbance input are used to characterize the nature of the feedforward/state feedback laws. Explicit synthesis formulas are derived that relate the optimal manipulated input to the system states and the measurable disturbance. It is shown that when the degree of singularity with respect to the manipulated input is infinite, the end‐point optimization problem reduces to a regulation problem in the presence of disturbances. For finite degrees of singularity, the optimal feedforward/state feedback law is, in general, dynamic. The proposed methodology is illustrated through several end‐point optimization problems in batch reactors.
Runaway phenomena in low‐density polyethylene autoclave reactors
Authors: Zhang Simon X. , Read Nolan K. , Ray W. Harmon 
Published: June 2004

AbstractTo study runaway behavior in autoclave low‐density polyethylene (LDPE) reactors, a kinetic model for a perfectly stirred tank reactor is presented. The kinetic model not only includes the standard initiation, propagation, and termination reactions for polymerization, but it also has free radical reactions that describe the decomposition of ethylene ultimately leading to a runaway. Dynamic simulation of the model indicates runaway behavior for the following conditions: excess initiator in feed; feed impurity; feed temperature disturbance; controller failure; and poorly tuned controller. Operating strategies such as mixed initiator feeds and grade transitions are also explored from a dynamic view. Stability analysis indicates safe operating limits for certain variables at typical conditions. The model provides useful insights for preventing runaway reactions in LDPE autoclaves.
PDF modeling of turbulent‐mixing effects on initiator efficiency in a tubular LDPE reactor
Authors: Tsai Kuochen , Fox Rodney O. 
Published: June 2004

AbstractThe effect of turbulent mixing in the reaction zone of a tubular low‐density polyethylene reactor was studied by combining a Lagrangian composition probability density function (LCPDF) code with a computational fluid‐dynamics code. Because the LCPDF code can treat the chemical reaction terms in a turbulent flow without resorting to moment closures, it is used to describe the temperature and scalar fields of reactants including initiator and monomer molar concentrations, and the moments of the molecular weight distribution. The chemical reaction terms are efficiently dealt with using a three‐parameter chemical lookup table that contains the temperature and composition changes as functions of initiator and monomer concentrations and temperature over a small time step. The reaction‐rate constants from the study of Lee and Marano (1979) are functions of temperature and pressure. The flow fields are obtained using the k – ε turbulence model. Because the temporal and spatial evolution of all fields in the reactor can be simulated, it is possible to study the effect of the initiator injection location, flow rate and temperature of the monomer and initiator feed streams on polymerization in considerable detail. Moreover, by observing the probability distribution of the composition fields, a better understanding of hot‐spot formation is achieved, leading to improved reactor designs.
Effect of axial gas dispersion on MTO light–olefin yield: Microreactor data
Authors: Tshabalala Samuel N. , Squires Arthur M. 
Published: June 2004

AbstractPulsed‐tracer experiments conducted for horizontal gas flow through a horizontal duct of rectangular cross section (height = 12.7 mm) containing powder in the coherent‐expanded vibrated‐bed state have been reported (Benge and Squires, 1995; Squires and Benge, 1995). Axial gas dispersion in the duct can be treated by a simple model: the dispersion is a “spreading” superimposed on plug flow, with no gas bypassing and no lagging gas indicative of dead zones. At a given weight hourly space velocity, a large variation in axial Peclet number (Peax) can be achieved by varying length of duct and gas superficial velocity in proportion, with no substantial effect upon gas–solid contacting efficiency. Herein, we report data for the methanol‐to‐olefin reaction conducted in three ducts at lengths affording Peax‐values from ∼ 2 to ∼ 20. The new data display a trend in light‐olefin yield vs. Peax similar to that reported earlier for three fluid‐bed reactors at diameters of 10, 102 and 600 mm.
Diffusion measurements through embedded zeolite crystals
Authors: Sun Matthew S. , Talu Orhan , Shah D. B. 
Published: June 2004

AbstractFor the first time, a macroscopic method was used to measure directly diffusivity of fast diffusing species such as methane in silicalite. A form of Wicke–Kallenbach technique was applied to measure intracrystalline diffusivity. The technique uses a single embedded zeolite crystal as a membrane and a mass‐selective detector to determine the transient mass response and hence the diffusion flux passing through the membranne. A concentration‐difference driving force is used rather than a pressure‐difference driving force. The diffusivities calculated from both adsorption and desorption transients were in excellent agreement, thus providing a check for the self‐consistency of the data. The diffusivities of carbon dioxide, methane, ethane, propane and butane in silicalite between 30 and 70°C have been compared with those from the literature. The diffusivities reported here are between those measured with microscopic methods and those measured with other macroscopic methods.
Feedback control of hyperbolic PDE systems
Authors: Christofides Panagiotis D. , Daoutidis Prodromos 
Published: June 2004

AbstractThis article deals with distributed parameter systems described by first‐order hyperbolic partial differential equations (PDEs), for which the manipulated input, the controlled output, and the measured output are distributed in space. For these systems, a general output‐feedback control methodology is developed employing a combination of theory of PDEs and concepts from geometric control. A concept of characteristic index is introduced and used for the synthesis of distributed state‐feedback laws that guarantee output tracking in the closed‐loop system. Analytical formulas of distributed output‐feedback controllers are derived through combination of appropriate distributed state observers with the developed state‐feedback controllers. Theoretical analogies between our approach and available results on stabilization of linear hyperbolic PDEs are also identified. The developed control methodology is implemented on a nonisothermal plug‐flow reactor and its performance is evaluated through simulations.
Chemical vapor deposition of carbon on graphite by methane pyrolysis
Authors: Bammidipati Sreekanth , Stewart Gregory D. , Elliott J. Richard , Gokoglu Suleyman A. , Purdy Mark J. 
Published: June 2004

AbstractChemical vapor deposition (CVD) of carbon by methane pyrolysis onto a nonporous graphite rod was studied experimentally in a hot‐wall, laminar‐flow reactor and theoretically by finite‐element modeling. Reactor pressure ranged from 10 to 40 torr, control temperature from 1,000 to 1,100°C, and methane was diluted with H2, Ar, or N2. The average residence time ranged from 1 to 50 s. A combination of experimental measurement and numerical modeling was used to accurately characterize fluid and thermal fields inside the reactor. Tractable numerical models were developed for the CVD of carbon with gas‐phase chemical reaction mechanisms based on accepted kinetic expressions, Simpler mechanisms were reduced from ones based on a more extensive list of intermediate reactions by systematic comparison to experimental data. A simplified but rational model is recommended based on the best fit to experimental data at various flow rates and pressures. The study indicates that axial dispersion plays a significant role and the pressure fall‐off effect for rate coefficients of unimolecular decomposition reactions must be taken into account.
Kinematic waves in the isothermal melt spinning of Newtonian fluids
Authors: Kim Byung Min , Hyun Jae Chun , Oh Joo Seok , Lee Seung Jong 
Published: June 2004

AbstractThe hyperbolic nature of the continuity equation in the melt spinning process causes several kinematic waves to travel in the spinline. Since different traveling waves possess different speeds, the system invariably has many characteristic times. This study computes the traveling times of these waves, finds their interrelationships, and then derives the criterion for the instability of periodic oscillation known as draw resonance. The period of oscillation is greater than the fluid residence time, but equal to the time during which both the maximum and minimum spinline cross‐sectional area waves travel in sequence from the spinneret to the take‐up. The onset of draw resonance occurs when two successive unity‐throughput waves are also able to travel the same distance within the same period of oscillation. This happens when the drawdown ratio takes on the critical value of 20.218 for the isothermal spinning of Newtonian fluids, agreeing with the value reported by earlier researchers.
Effect of droplet size on the rheology of emulsions
Authors: Pal Rajinder 
Published: June 2004

AbstractThe effect of droplet size on the rheological behavior of water‐in‐oil and oil‐in‐water emulsions was investigated using a controlled‐stress rheometer. Results indicate that the droplet size has a dramatic influence on emulsion rheology. Fine emulsions (water‐in‐oil or oil‐in‐water) have much higher viscosities and storage moduli than the corresponding coarse emulsions. The shear‐thinning effect is much stronger in the case of fine emulsions. When coarse droplets are replaced by fine droplets (keeping total volume fraction of the dispersed phase constant), the resulting emulsion exhibits a minimum in rheological properties (viscosity, storage and loss moduli, time constant) at a certain proportion of fine droplets. However, the minimum in viscosity occurs only at low shear stresses. At high stresses, the viscosity of the mixed emulsion increases as the proportion of fine droplets increases. The study of the aging effect on the rheological behavior shows that water‐in‐oil emulsions age much more rapidly than the oil‐in‐water emulsions.
Comparison of isotherm models for hydrocarbon adsorption on activated carbon
Authors: Malek A. , Farooq S. 
Published: June 2004

AbstractA number of equilibrium isotherm models can be utilized for correlating experimental equilibrium results. Seven different isotherm models are studied using equilibrium data of methane, ethane and propane in activated carbon. Besides comparing the goodness of data fit, the limiting behaviors as well as the pressure and temperature derivatives of the equilibrium isotherm models are also investigated. Performance of multicomponent extensions of these isotherm models and their combinations with the ideal adsorbed solution theory are also compared with experimental data. This systematic evaluation of the more important equilibrium isotherm models provides the general basis for making a preliminary selection of an effective model for a given application. Although an accurate and thermodynamically consistent model is desirable, these requirements may often be compromised for computational simplicity in dynamic process modeling studies.
Kinetics of ozone photolysis in aqueous solution
Authors: Gurol Mirat D. , Akata Aysegul 
Published: June 2004

AbstractThe kinetics of ozone photolysis were investigated experimentally in well‐characterized aqueous solution and by a conceptual kinetic and mechanistic model developed based on possible reaction pathways. The primary quantum yield of ozone photolysis at 245‐nm wavelength was measured at 20°C as 0.48 ± 0.6. The rate of ozone photolysis increased with increasing light intensity, ozone concentration and pH, and decreased with increasing inorganic carbon concentration in water. The model was able to predict the experimentally observed rate of ozone photolysis quite well for various combinations of UV light intensity, initial ozone concentration and water characteristics. In the mechanistic model, the formation of the hydroxyl radical, which is the primary oxidant in the O3/UV process, was tied to the decomposition of ozone; therefore, the model can now be extended to predict the oxidation rates of water contaminants by the hydroxyl radical generated in the O3/UV process.