bluuebunny/crossref_metadata_2025_split huggingface dataset:
Racemic leucine separation by hollow‐fiber extraction
Authors: Ding H. B. , Carr P. W. , Cussler E. L. 
Published: June 2004

AbstractRacemic leucine can be separated into d‐ and l‐isomers by fractional extraction across microporous hollow fibers. In this extraction, an aqueous solution of the racemate is fed to the lumen of the fibers, and an octanol solution of dodecyl‐l‐hydroxyproline flows countercurrently outside of the fibers. The interface between feed and extractant is stabilized by filling the pores in the hollow‐fiber walls with a cross‐linked polyvinylalcohol gel which offers negligible resistance to mass transfer. The extraction with dodecyl‐l‐hydroxyproline deliberately imitates earlier studies, facilitating comparisons of hollow‐fiber extraction with other techniques. The results show that the isomer yield per equipment volume of racemic separation is 100 times greater than that in a continuously rotating extractor, and 1,000 times greater than that in a conventional packed tower.
Optimal grade transitions in a gas phase polyethylene reactor
Authors: McAuley K. B. , MacGregor J. F. 
Published: June 2004

AbstractUsing gas‐phase technology many grades of polyethylene can be produced in a single reactor. For a series of three polyethylene products, model‐based dynamic optimization is used to determine optimal grade changeover policies. Optimal manipulated variable profiles are determined for hydrogen and butene feed rates, reactor temperature setpoint, gas bleed flow, catalyst feed rate, and bed level setpoint. It is shown that large transitions in melt index are hampered by slow hydrogen dynamics, and that the time required for such a transition can be reduced by manipulating the temperature setpoint and the bleed stream flow. Reduction of the bed level and catalyst feed rates during changeovers can significantly reduce the quantity of off‐specification polymer produced. It is demonstrated that melt index and density are not sufficient to characterize the properties of polymer produced during grade transitions, and that the shape of the cumulative copolymer composition distribution is very sensitive to the grade changeover policy used. Optimal transition policies should not be implemented without feedback control. Disturbances and model mismatch can result in product property trajectories which deviate significantly from the nominal optimal trajectory.
Unsteady axial mixing by natural convection in a vertical column
Authors: Baird M. H. I. , Aravamudan K. , Rao N. V. Rama , Chadam J. , Peirce A. P. 
Published: June 2004

AbstractUnsteady axial mixing due to addition of a batch of sodium chloride solution at the top of a water‐filled tube (2.63 cm i.d.) has been studied by measuring the developing concentration profile and the advancing front with dye added to the brine. Data have also been obtained with added baffle plates, with the use of a viscous aqueous solution, and in smaller diameter (1.48, 1.91 cm) tubes. Results can be approximately correlated by means of a model based on unsteady one‐dimensional turbulent dispersion. Laminar flow affects the behavior of the advancing front at which the salt concentration is lowest.
Modeling of growth rate dispersion in batch cooling crystallization
Authors: Bohlin Martin , Rasmuson Åke C. 
Published: June 2004

AbstractThe influence of growth rate dispersion on the product‐size distribution of batch cooling crystallization is investigated by computer simulations. The model accounts for primary and magma density‐dependent secondary nucleation, and growth rate dispersion of the constant crystal growth type. The model is solved by a combination of the method of characteristics and moment analysis, by which the entire product‐size distribution is recovered. The study includes three different growth rate activity distributions, and the influence of the corresponding coefficient of variation is analyzed for unseeded and seeded processes. The results show that the effect of growth rate dispersion on the crystal‐size distribution may be significant even at moderate dispersion. At high dispersion, even the actual shape of the growth rate activity distribution may become important.
Gas holdup in stacked expanded metal sheets
Authors: Piovano Stella , Böhm Ursula , Franco Juan I. 
Published: June 2004

AbstractThe gas holdup in different arrays of expanded metal sheets was determined by the conductimetric method.A model had to be developed for analyzing the conductimetric behavior of the cell in the presence of the metallic sheets. The validity of this model has been verified with single phase flow. The application to two‐phase flow allowed the obtaining of information on gas accumulation within stacked expanded metal sheets, showing a strong influence of the geometry of the array. Depending on the orientation of the stacked sheets and of the large dimension of the expanded metal, the gas holdup within the packing will be equal or larger than in an empty bubble column.
Startup of an industrial adiabatic tubular reactor
Authors: Verwijs J. W. , van den Berg H. , Westerterp K. R. 
Published: June 2004

AbstractThe dynamic behaviour of an adiabatic tubular plant reactor during the startup is demonstrated, together with the impact of a feed‐pump failure of one of the reactants. A dynamic model of the reactor system is presented, and the system response is calculated as a function of experimentally‐determined, time‐dependent, manipulated variables. The values of model parameters are estimated by using the SimuSolv (1991) computer program. The data set collected during the reactor start‐up is used for the parameter estimation procedure. An excellent agreement is obtained between the experimental and the calculated system response. Many continuously‐operated commercial reactors require a complete conversion of one of the main reactants at the reactor exit. It is shown that for an industrial tubular reactor a much higher initial reactor temperature is required during the startup, compared to the reactor inlet temperature during normal steady‐state operation, to ensure a complete reactant conversion. Much more research is necessary to determine whether this is a generally valid rule.
Slow motion of multiple droplets in arbitrary three‐dimensional configurations
Authors: Keh Huan J. , Tseng Yi K. 
Published: June 2004

AbstractThis article presents a combined analytical‐numerical study for the hydrodynamic interactions of an arbitrary finite cluster of spherical droplets at low Reynolds numbers. The droplets may differ in radius, in viscosity and in migration velocity. The theory developed is the most general solution to the mobility or resistance problem of slow motion of an assemblage of fluid spheres in a three‐dimensional unbounded medium. Using a boundary collocation technique, the creeping flow equations are solved in the quasisteady situation, and the interaction effects among the droplets are evaluated for various cases. For the motion of two‐droplet systems, our results for the droplet interaction parameters at all orientations and separation distances agree very well with the exact solutions obtained by using spherical bipolar coordinates or the asymptotic solutions obtained by using the connector algebra. The mobility parameters of linear chains of three droplets have been calculated, demonstrating that the existence of the third sphere can significantly affect the mobility of the other two spheres. The mobility results are also presented for the motion of chains containing up to 101 identical and equally spaced spheres, and the end effect exhibited by a chain of spheres can be found. Finally, our “exact” solutions for the hydrodynamic interaction between two fluid or solid spheres are used to find the effect of volume fractions of particles of each type on the mean settling velocities of the particles in a bounded suspension.
Circumferential tube skin temperature profiles in thermal cracking coils
Authors: Heynderickx G. J. , Cornelis G. G. , Froment G. F. 
Published: June 2004

AbstractUniform temperature distributions in cracking furnaces favor the run length of the production cycle of the furnace and the tube metal life. The coupled simulation of the furnace and the reactor tubes by means of the zone method is extended to permit the calculation of circumferential nonuniformities under reaction conditions. Circumferential tube skin temperatures were found to vary over 50°C and more, due to “shadow effects.” As a result, nonuniform coking rates and coke layers are obtained. The model and computational scheme presented here can be used as a guide for plant operators and as a tool for designing.
Flow of dilute gas‐particle suspensions
Authors: Rhodes Martin , Zhou Shihui , Benkreira Hadj 
Published: June 2004

AbstractA semi‐empirical model of the radial segregation of solids in upward flow of dilute gas‐particle suspensions in riser systems is presented on the basis of a reduced form of the fundamental two‐phase flow governing equations and experimental evidence concerning the solids concentration at the wall. The following simple expression for the radial solids concentration profile is obtained:

and is in agreement with experimental data over a wide range of operating conditions: superficial gas velocity from 1.4 to 15.3 m/s, riser diameter from 0.032 to 0.40 m, imposed solids flux from 6.60 to 207 kg/m2·s, mean particle size from 32 to 120 μm and particle density from 1,000 to 3,500 kg/m3.The model confirms the existence of the core‐annulus flow structure in gas‐particle suspensions reported in riser reactors, circulating fluidized beds, and the freeboard of bubbling fluidized beds.
Analytical and knowledge‐based redundancy for fault diagnosis in process plants
Authors: Fathi Zohreh , Ramirez W. Fred , Korbicz Jozef 
Published: June 2004

AbstractThe increasing complexity of process plants and their reliability have necessitated the development of more powerful methods for detecting and diagnosing process abnormalities. Among the underlying strategies, analytical redundancy and knowledge‐based system techniques offer viable solutions. In this work we consider the adaptive inclusion of analytical redundancy models (state and parameter estimation modules) in the diagnostic reasoning loop of a knowledge‐based system. This helps overcome the difficulties associated with each category. The design method is a new layered knowledge base that houses compiled/qualitative knowledge in the high levels and process‐general estimation knowledge in the low levels of a hierarchical knowledge structure. The compiled knowledge is used to narrow the diagnostic search space and provide an effective way of employing estimation modules. The estimation‐based methods that resort to fundamental analysis provide the rationale for a qualitatively‐guided reasoning process. The overall structure of the fault detection and isolation system based on the combined strategy is discussed focusing on the model‐based redundancy methods which create the low levels of the hierarchical knowledge base. The system has been implemented using the condensate‐feedwater subsystem of a coal‐fired power plant. Due to the highly nonlinear and mixed‐mode nature of the power plant dynamics, the modified extended Kalman filter is used in designing local detection filters.
In‐situ measurement of interfacial pH using a rotating ring‐disk electrode
Authors: Hessami Sholeh , Tobias Charles W. 
Published: June 2004

AbstractA nonintrusive technique for the in‐situ measurement of interfacial pH and current efficiency has been developed. A ring electrode, operated potentiometrically at opencircuit, is used to measure the pH change at a rotating disk electrode. The technique takes advantage of the well‐characterized hydrodynamics at the rotating disk electrode and has the added advantage that the pH probe, the ring electrode, is not interfering with the flow field and the current distribution on the disk. To determine the pH at the disk electrode by measuring the potential of the ring, the radial transport of hydronium ions across the insulating gap and on the ring is analyzed taking into account the effect of homogeneous dissociation reactions of water and metal‐hydroxide ion complexes. Shifts in the ring potential caused by hydrogen super‐saturation and ohmic drop are also evaluated. A platinized ring electrode in a hydrogensaturated electrolyte is shown to provide a stable and reproducible hydrogen ion sensor with a Nernstian response to the changes in the bulk pH. Performance of the ring is evaluated by generating hydrogen at the disk electrode from a dilute acid solution, in the absence of other electrochemical reactions. The technique is then applied to determine the interfacial pH of nickel, iron and nickel‐iron alloy electrodeposition with concurrent hydrogen evolution. This method was also used to measure the current efficiency of nickel electrodeposition in a fast, nonintrusive and in‐situ manner.
Circulation model for absorption and dispersion in cocurrent bubble columns
Authors: Rice Richard G. , Geary Nicholas W. , Burns Louis F. 
Published: June 2004

AbstractLiquid circulation and a finite entrance zone are accounted for in modeling gas absorption into cocurrent flows of pure oxygen and CMC solutions. The new models lead to a Taylor‐like correction factor to account for circulation in both absorption and transient dispersion experiments. Steady‐state experiments for viscosities of 1, 12, 25 and 50 mPa·s were conducted by measuring dissolved gas concentration profiles along the column axis. Liquid mixing experiments were performed using the transient acid‐base neutralization method, leading to measured values of axial dispersion coefficients which depended on viscosity to the power 0.75. Measurements suggest that circulation was minuscule owing to uniform gas injection by Flexisparger in the present column.
Effect of energy distribution on sorption kinetics in bidispersed particles
Authors: Hu Xijun , Rao Gade N. , Do Duong D. 
Published: June 2004

AbstractIn this article, we present a model to describe sorption kinetics of gaseous adsorbates onto bidispersed heterogeneous microporous particles. This model describes adsorption and desorption of gaseous adsorbates with allowance for the energy distribution of adsorption site for both equilibrium isotherm and diffusion of the adsorbed species. The adsorbed species of all energy level diffuse in two directions inside the particle. One direction is the direction along the particle coordinate while the other is the direction perpendicular to the particle coordinate (that is, the direction into the grain of the solid). This model is more fundamental than models previously proposed by Do and coworkers. Effects of energy distribution on the adsorption and desorption dynamics are investigated. Extensive experimental analysis of adsorption and desorption dynamics of ethane and propane onto Ajax activated carbon are carried out to validate this theory.
Model predictive control with linear models
Authors: Muske Kenneth R. , Rawlings James B. 
Published: June 2004

AbstractThis article discusses the existing linear model predictive control concepts in a unified theoretical framework based on a stabilizing, infinite horizon, linear quadratic regulator. In order to represent unstable as well as stable multivariable systems, the standard state‐space formulation is used for the plant model. The incorporation of a nominally stabilizing constrained regulator eliminates the current requirement of tuning for nominal stability. Output feedback is addressed in the well‐established framework of the linear quadratic state‐estimation problem. This framework allows the flexibility to handle nonsquare systems, noisy inputs and outputs, and nonzero input, output, and state disturbances. This formulation subsumes the integral control schemes designed to remove steady‐state offset currently in industrial use. The online implementation of the controller requires the solution of a standard quadratic program that is no more computationally intensive than existing algorithms.
Dispersion coalescence: Kinetic stability of creamed dispersions
Authors: Lobo Lloyd , Ivanov Ivan , Wasan Darsh 
Published: June 2004

AbstractA Model is developed to predict the coalescence behavior in liquid‐liquid dispersions. Coalescence times are based on the lifetimes of the single films that are formed between adjoining drops in a creamed or sedimented dispersion. The model is developed for smaller drop sized dispersions (<100 μm) in which the drops are not substantially deformed due to gravitational forces. The model predicts that coalescence occurs between the emulsion drops without the gross separation of the dispersed phase (that is, the dispersion does not separate into distinct oil and water phases). As a result of interdrop coalescence, the mean drop size of the dispersed phase increases along with an increase in the polydispersity of the drop size. The model's predictions are used to develop a quantitative relationship between the lifetime of the single film and the rate of increase of the mean drop size of the dispersion. The model also accounts for the effect on dispersion stability via the models of single film drainage in the presence of surfactant.
Adaptive nonlinear cell mass state estimator for a continuous yeast fermentation
Authors: Ramseir Michael E. , Agrawal Pramod , Mellichamp Duncan A. 
Published: June 2004

AbstractOn‐line measurement of the important state variables in fermentations, particularly cell mass concentration, remains a difficult problem. However, a number of secondary or environmental variables can be measured conventionally and on‐line, such as pH, and CO2 and O2 in the exhaust gas. Stephanopoulos and San (1984) have developed a modeling approach, based on species balances, that provides relations between the environmental and important state variables. Using such a model, the important state variables can be estimated in principle from more easily accessible on‐line measurements.In this article, a new adaptive estimator is developed, incorporating as its basis an underlying nonlinear model so as to utilize the best possible a priori process knowledge. Base addition rate and CO2 offgas concentration are measured on‐line and periodically. Cell mass measurements are incorporated infrequently and even at irregular sampling periods, thus providing a very flexible scheme. Only a single adapted parameter is required to match the model to the plant operating characteristics. This simple but rigorous model form results in an estimator that is easy to implement and to tune and which exhibits long‐term robustness due to its multirate feedback structure. Experimental results from a laboratory‐scale continuous fermentor show that such a cell mass estimation scheme yields excellent performance both open‐loop (without control) and as a part of conventional and nonlinear adaptive control approaches.
Memory‐integral mass‐transfer models for adsorption process simulation
Authors: Harriott George M. 
Published: June 2004

AbstractThe memory integral based on convolution of the unit step response to loading a clean pellet with concentration history at the pellet surface is proposed for computations of mass transfer in adsorption process simulation. Although rate laws are widely used to describe diffusion and sorption in pellets for slow (Glueckauf, 1955) and moderate (Kim, 1989) rates of mass transfer, these models fail to describe mass transfer at short contact times because the concentration history experienced by the pellet is not accounted for. To facilitate memory integral computations, approximations to the unit step response to loading a clean pellet are derived based on a single moving finite element and by asymptotic matching in time. Numerical evaluation of the memory integral is demonstrated on simple cycles which show the merits of this approach.
Surface diffusion of multilayer adsorbed species
Authors: Chen Y. D. , Yang R. T. 
Published: June 2004

AbstractA model based on kinetic theory is derived for surface diffusion of multilayer adsorbed species on a homogeneous surface. A simple solution is obtained for expressing the concentration dependence of surface diffusivity. For monolayer adsorbate, the solution is reduced to the classical HIO model. This solution, in principle, requires no fitting parameters and is capable of predicting the unique concentration dependent behavior exhibited by multilayer surface diffusion, where the diffusivity first increases with surface concentration to a maximum followed by a decrease. Seventeen sets of experimental data from the literature are used to test the model, with fair results. It is also shown that heats of adsorption can be calculated from multilayer surface diffusivity data by using this model.
Polymer‐solvent diffusion and equilibrium parameters by inverse gas‐liquid chromatography
Authors: Romdhane Ilyess Hadj , Danner Ronald P. 
Published: June 2004

AbstractThe solubility and diffusion coefficients of low‐molecular‐weight penetrants in a polymeric stationary phase were determined by use of inverse gas chromatography (IGC). Previously reported applications of packed‐column IGC to the measurement of the stationary‐phase diffusion coefficient have been carried out via the van Deemter approach. We present here a new mathematical model to describe the chromatographic process in packed columns. A moment analysis procedure was developed from this model to analyze the chromatographic peaks eluting from packed columns. The first moment (mean retention time) is determined to depend only on the equilibrium partition coefficient, whereas the second moment (variance) is related to both the thermodynamic and transport properties of the polymer‐solute system. The validity of the technique was demonstrated by measuring the diffusivity and solubility of toluene and benzene in polystyrene above the glass transition temperature of the polymer. Free‐volume theory was used to interpret and correlate the diffusivity data obtained at infinite dilution of the solute. One can use the results obtained at infinite dilution to predict reliably via the free‐volume theory the concentration and temperature dependence of polymer‐solvent diffusion coefficients.
Mean velocity and turbulence fields inside a β=0.50 orifice flowmeter
Authors: Morrison Gerald L. , Deotte Robert E. , Nail Gregory H. , Panak David L. 
Published: June 2004

AbstractThe flow field inside an orifice flowmeter with a beta ratio of 0.50 operating at a Reynolds number of 91,100 has been studied using a three‐color, 3‐D laser Doppler anemometer system. Mean velocity measurements show large radial velocities leading into the orifice, the fluid separating from the orifice plate at the throat, the presence of a vena contracta farther downstream, flow reattachment to the pipe wall 5.3 pipe radii (R) downstream, the presence of a small upstream recirculation zone, and both a primary and secondary recirculation zone downstream of the orifice plate. The static wall pressure distribution attains a minimum pressure at 1.5 R, which does not coincide with the location of the vena contracta (0.75 R). Distributions of the entire Reynolds stress tensor are presented along with calculated values of turbulence kinetic energy, turbulence kinetic energy production, vorticity, and turbulence induced accelerations. These data are analyzed to interpret the complex turbulent flow field.
Thermal method of measuring concentration during turbulent reactive mixing
Authors: Singh Madan , Toor H. L. 
Published: June 2004

AbstractWhen the total reactant diffusivities are equal, the time‐average concentration fields which result from turbulently mixing reactants can be measured thermally if the temperature difference between the feed streams is suitably chosen. Each reactant profile is obtained from measurements of the temperature profile in an experiment in which the inlet temperature of that reactant is less than that of the other reactant by an amount equal to the adiabatic reaction temperature change of the measured reactant. The method is demonstrated in a liquid phase confined jet reactor.