bluuebunny/crossref_metadata_2025 huggingface dataset:
Transport of n‐paraffins in zeolite T
Authors: Magalhães Fernão D. , Laurence Robert L. , Conner William Curtis
Published: June 2004

Mass transport of n‐heptane, n‐nonane, and n‐dodecane in zeolite T was studied using a gravimetric uptake technique. The sigmoidal shape of the uptake curves indicated the existence of a nondiffusional resistance in the transport process. To compute the transport parameters, an approach based on the temporal moments of the uptake curves was developed. A model involving an external resistance dependent on the local slope of the sorption isotherm best represented the data. No evidence of the “window effect” reported by Gorring was found. The measured diffusivities decrease with chain length, while the activation energies remain essentially constant. Both resistances (diffusional and nondiffusional) depend strongly on lattice loading, and their relative contributions for the transport time change as saturation is approached.
Effects of isomorphic compounds on the purity and morphology of L‐isoleucine crystals
Authors: Koolman Hannia Canossa , Rousseau Ronald W.
Published: June 2004

L‐Isoleucine and the isomorphic amino acids L‐leucine and L‐valine serve as a model system in the examination of the effects of impurities on the purity and morphology of crystals recovered from batch crystallizations. Factors are determined that influence crystal purity and, concomitantly, crystal size and shape. Characteristics of the L‐isoleucine crystal are determined by X‐ray analysis and compare favorably with the literature. These and literature characteristics of L‐leucine and L‐valine are used in simulations to evaluate possible mechanisms by which lattice substitutions with impurities lead to size and morphological changes.
Scanning force microscopy of gypsum dissolution and crystal growth
Authors: Hall Christopher , Cullen David C.
Published: June 2004

Dissolution and crystal growth on the (010) cleavage surface of mineral gypsum have been studied by scanning force microscopy, which allows observation of surfaces while they are in contact with aqueous solutions. Etch pits formed on the (010) surface in contact with solutions undersaturated with respect to gypsum are oriented in relation to the [001] zone axis. Edges parallel to [001] are stable or move only slowly. Edges parallel to [100] and [101] move more rapidly. Crystal growth by migration of step edges is observed in contact with supersaturated solutions. The inhibition of crystal growth by phosphonate produces gross changes in the texture of steps and terraces.
Model for flue‐gas desulfurization in a circulating dry scrubber
Authors: Neathery James K.
Published: June 2004

A simple model was developed to describe the absorption of SO2 in a circulating dry scrubbing (CDS) process, which is a semidry, lime‐based, flue‐gas desulfurization (FGD) process that utilizes a circulating fluidized bed arrangement for contacting a sorbent with SO2‐laden flue gas under “coolside” conditions. The reaction chemistry is thought to be similar to that of spray‐drying absorption. The liquid‐phase mass‐transfer coefficient was successfully modeled as a function of the sorbent particle spacing on the wetted surfaces. Gas‐phase mass‐transfer resistances were assumed to be insignificant. Due to the high surface area available in a CDS reactor, the evaporation rate of water from the slurry was modeled as “constant‐rate” drying according to classic spray‐dryer theory. However, the “falling‐rate” and “diffusion” evaporation stages were negligible in CDS since sorbent particle bunching at the surface of the slurry is nonexistent.
Separation of branched hexane isomers using zeolite molecular sieves
Authors: Huddersman K. , Klimczyk M.
Published: June 2004

A range of small, medium and large pore zeolite, and their modified forms are studied for their ability to separate di‐ from monobranched isomers of hexane. The separation studies are carried out using high‐temperature (250–350°C) gas chromatography. Beta(H,Ba) is found to be the most effective separator of 2,3‐dimethylbutane and 3‐methyl‐pentane and is therefore studied for its sorption capacities toward the two hexane isomers.
Neural network architecture for process control based on the RTRL algorithm
Authors: Chovan Tibor , Catfolis Thierry , Meert Kürt
Published: June 2004

Neural‐network‐based control schemes are generally designed by replacing standard elements of the classic control schemes by feedforward neural networks. The introduction of discrete time recurrent networks, which are inherently dynamic systems, into those schemes can simplify the design of neural controllers. The concept of applying recurrent networks in indirect adaptive control schemes is described. A combined network cluster consisting of the control network and the model network is constructed to allow the use of the real‐time recurrent learning algorithm. To demonstrate the feasibility of the method two simulation examples are presented.
Experimental evaluation of designs for the simulated countercurrent moving bed separator
Authors: Tonkovich Anna Lee Y. , Carr Robert W.
Published: June 2004

Simulated countercurrent chromatography is a continuous flow method for separation of binary mixtures or for separation of multicomponent mixtures into two fractions. Countercurrent flow is simulated by moving the feed point past several fixed adsorbent beds at a rate between the single column breakthrough time of each component. The number of columns and the location of the inlet and outlet port were varied to investigate product purity and productivity. Concentration profiles of the effluent product streams were measured. The maximum product concentration exceeds the feed concentration during part of the feed‐switching cycle and drops to zero during part of the cycle. Three configurations were tested with the total number of columns varying between three and eight. For the test separation chosen, gaseous propylene and dimethyl ether on Chromosorb 101, high purity (> 99%) product streams were obtained with an optimal four‐column configuration using three desorbent streams.
Process identification using discrete wavelet transforms: Design of prefilters
Authors: Palavajjhala Srinivas , Motard Rodolphe L. , Joseph Babu
Published: June 2004

Most advanced control applications rely on good dynamic process models. The performance of the control system depends on the accuracy of the model used. Typically, such models are developed by conducting off‐line identification experiments on the process. These identification experiments often result in input–output data with small output signal‐to‐noise ratio, and using these data results in inaccurate model parameter estimates. Prefilters are used to separate useful information from the noise in the input–output data and to improve parameter estimates. A systematic design procedure for selecting a prefilter using discrete wavelet transforms is presented. The design procedure provides explicit information on the compromises in prefilter design, interpreted in terms of parameter variance and bias. The prefilter design procedure is then applied to identify a second‐order output error model.
Robust Smith‐predictor controller for uncertain delay systems
Authors: Lee T. H. , Wang Q. G. , Tan K. K.
Published: June 2004

Necessary and sufficient conditions for the robust stability and performance of the Smith‐predictor controller, modeled under a norm‐bounded uncertainty, are given in a general formulation. In addition, a practical stability condition is obtained as a special corollary of the main results. These conditions provide useful and practical guidelines for the development of a systematic robust design method. In particular, an application is developed for the robust control of first‐order deadtime systems with simultaneous uncertainties in all three parameters of the model. A simulation example and the results of a case study on the robust level control of a coupled‐tanks apparatus are provided for illustration.
Reaction of methanol with chlorate ions in acid solution containing Hg+2 by NMR
Authors: Ernst William R. , Indu Bhart , Crump Brian , Gelbaum Leslie T.
Published: June 2004

The reaction rate of methanol was measured in solutions of sodium chlorate and sulfuric acid at several levels of temperature and concentration, in the presence of mercuric nitrate. The progress of the reaction was monitored by proton NMR signals corresponding to methanol and formic acid. Chlorine dioxide formation was suppressed by adding mercuric nitrate, which was shown earlier to catalyze the disproportionation of the intermediate species, chlorous acid, and sequester chloride ions. The reaction is first order in methanol and chlorate concentration and in the Hammett acidity function. The reaction of formic acid, sodium chlorate and sulfuric acid was also studied using the same technique. Formic acid was stable and did not react with chlorate at a measurable rate, even at concentrations and temperatures of a commercial process. This study relates to commerical processes that produce chlorine dioxide by reducing chlorate ions with methanol.
Closed‐form solution for the reconstruction problem in porous media
Authors: Giona Massimiliano , Adrover Alessandra
Published: June 2004

The statistical reconstruction of lattice models of real porous media is one of the basic engineering problems in the theory of porous structure and has a variety of applications in the study of transport, in mineral processing, and in material characterization. A systematic analysis of the reconstruction problem of porous media is presented, as well as a closed‐form solution. The solution is presented in the form of a linear filter acting on Gaussian processes by means of a superposition of elementary correlated processes with prescribed correlation properties, and in the form of a memoryless process recalling the theory of Khinchin on the properties of correlation functions. The connection of this approach with models of correlated percolation is also discussed.
Asymptotic analysis of die flow for shear‐thinning fluids
Authors: Weinstein Steven J. , Ruschak Kenneth J.
Published: June 2004

The flow in a single‐cavity extrusion die is considered by using 1‐D equations. Since the fluid flow in a well‐performing die does not deviate much from the case of perfect widthwise distribution, linearization of 1‐D governing equations is often justified. It is demonstrated how linearization can lead to significant simplifications in the analysis of die flow. A self‐consistent asymptotic expansion is identified to accomplish this linearization; the expansion can be generalized to obtain higher‐order corrections if necessary. In contrast to previous analyses, which have generally been numerical, analytic solutions are obtained to predict widthwise flow nonuniformities in a die having a specific cavity area variation and in which cavity inertial effects are important; these solutions are valid for power‐law and Newtonian fluids. A novel truncated power‐law approach to modeling the flow of shear‐thinning fluids in a die is proposed that combines, by using analytical criteria, the solutions for purely Newtonian or power‐law flow in the cavity and slot. The validity of this approach is demonstrated through comparison with numerical results of a 1‐D die model for a Carreau constitutive equation.
Maximum stable drop diameter in stirred dispersions
Authors: Lam Andrew , Sathyagal A. N. , Kumar Sanjeev , Ramkrishna D.
Published: June 2004

Transient drop‐size distributions of stirred dispersions undergoing breakage were experimentally measured at long stirring times. The results show that drops continue to break the entire duration of the experiment (at least up to 10 h) and force a reevaluation of the widely held concept of a maximum stable drop diameter, dmax. Transient distributions show the existence of self‐similarity, which is the same as that observed in transient distributions obtained at short times (up to ∼ 1 h, Sathyagal et al., 1995b) indicating that the nature of breakage does not change with time. The existence of similarity at long stirring times can be used to obtain good estimates of breakage rates of small drops by an inverse problem procedure.
Particles flow pattern and local heat transfer around tube in moving bed
Authors: Takeuchi Hiromi
Published: June 2004

Flow visualization of flowing particles around a tube of various types of tube arrangements in a moving bed was studied using X‐ray video films to obtain a relation between particles behavior and local heat‐transfer coefficients. A stagnant part of solid particles was observed on the tube in the case of a staggered arrangement. This part did not appear in the case of the single tube and the single row of tubes. The measured local heat‐transfer coefficients around a tube was decreased in this stagnant part. Furthermore, influences of different tube arrangements both on flow patterns of particles and on local heat‐transfer coefficients between tubes and bed were examined.
Codeposition on hot CVD surfaces: Particle dynamics and deposit roughness interactions
Authors: Tandon Pushkar , Rosner Daniel E.
Published: June 2004

To capture in a tractable manner essential coupling effects in CVD systems when particles generated in thermal boundary layers also deposit, a film theory was developed that predicts simultaneous vapor and particle deposition rates at a hot deposition surface. The codeposition rate prediction method also calculates for the first time the corresponding solid deposit roughness using recently published results of particle‐level simulations. For the numerical illustrations, the growth of TiO2(s) films by the codeposition of titanium tetra‐isopropoxide vapor and film‐nucleated/grown TiO2 particles (generated in the thermal boundary layer) was considered. Experimental rate data for this system are available. The continuum and particle‐level simulation methods provide: the interplay of vapor precursor kinetics, particle nucleation, growth, coagulation and diffusion in determining the complex “structure” of such multiphase chemically reacting boundary layers; wall deposition rates of both surviving vapors and film‐nucleated particles; and the “self‐consistent” microstructure (surface roughness) of the resulting solid deposit. Timely and tractable generalizations are discussed in the light of recent results for the transport properties and stability of “fractal‐like” aggregated particles.
Measuring vapor‐liquid equilibrium for aqueous‐organic systems: Review and a new technique
Authors: Turner Louis H. , Chiew Yee C. , Ahlert Robert C. , Kosson David S.
Published: June 2004

The vapor–liquid equilibrium (VLE) of aqueous–organic mixtures is important in understanding the physical chemistry behind the contamination and remediation of contaminated surface water, ground water and unsaturated soils. The state of the science is assessed, and available VLE and solubility data are reviewed comprehensively for several organic compounds. Benzene, toluene, o‐, m‐, p‐xylene, ethylbenzene, chloroform, 1,1,1‐trichloroethane (TCA), and trichloroethylene (TCE) were included in this study. Correlations are provided for estimation of VLE as a function of temperature. A new VLE apparatus was developed to extend experimental techniques found in the literature. Partitioning data for several aqueous–organic systems were measured. The collected data compared very favorably to data available in the literature. The new procedure is capable of providing high‐quality data over a wide range of liquid concentrations and temperatures.
Heat and mass transfer along a wetted porous plate in an air stream
Authors: Belhamri A. , Fohr J.‐P.
Published: June 2004

This experimental study examines 2‐D effects on long plate drying due to heat and moisture transfer through boundary layers and porous medium interaction. Many innovative techniques are used to measure accurately boundary layers (velocity, temperature, moisture), the wall surface (infrared sensors for temperature and moisture), and the medium (water content). Gradual decrease in surface saturation measured for several kinds of materials shows that their behavior is more complex than described by the Suzuki–Maeda model, which does not predict the mass exchange sufficiently. Based on the analysis of the transfer between the surrounding air and the porous medium at the scale of the pore on a rough surface, a flow classification is defined and other characteristics are proposed. The drying rate depends on the mass‐transfer coefficient, which varies along the plate with the air flow. The study of boundary layers gives the corresponding profiles with an acceptable accuracy. The laminar‐to‐turbulent transition increases mass‐transfer rates resulting in two drying fronts. The diffusive model proposed here predicts a gradual change in the drying rate along the plate similar to the experimental one.
Two‐dimensional model for circulating fluidized‐bed reactors
Authors: Schoenfelder H. , Kruse M. , Werther J.
Published: June 2004

A two‐dimensional reactor model for circulating fluidized beds (CFB) was studied based on the assumption that at every location within the riser, a descending dense phase and a rising lean phase coexist. Fluid mechanical variables may be calculated from one measured radial solids flux profile (upward and downward). The internal mass‐transfer behavior is described on the basis of tracer gas experiments. The CFB reactor model was tested against data from ozone decomposition experiments in a CFB cold flow model (15.6‐m height, 0.4‐m ID) operated in the ranges 2.5–4.5 m/s and 9–45 kg/(m2·s) of superficial gas velocity and solids mass flux, respectively. Based on effective reaction rate constants determined from the ozone exit concentration, the model was used to predict the spatial reactant distribution within the reactor. Model predictions agreed well with measurements.
Hierarchical data analysis of a replicate experiment in emulsion terpolymerization
Authors: Dubé Marc A. , Penlidis Alexander
Published: June 2004

A systematic study of the terpolymerization of butyl acrylate/methyl methacrylate/vinyl acetate (BA/MMA/VAc) was conducted. Complete replication of an emulsion terpolymerization experimental run was performed in a 5‐L stainless‐steel batch pilot‐plant reactor. The polymers produced were characterized for conversion, terpolymer composition, molecular weight averages, and average particle size. A hierarchical (nested) data analysis was performed on the experimental data to ascertain various sources of error and to allow for comments to be made regarding the data quality.
Catalytic supercritical water oxidation: Stability of Cr2O3 catalyst
Authors: Aki Sudhir N. V. K. , Ding Zhong‐Yi , Abraham Martin A.
Published: June 2004

Supercritical water oxidation has become a well‐known treatment technology for conversion of aqueous wastes. By introducing a catalyst, higher conversion and selectivity to carbon dioxide can be achieved at a lower temperature and at shorter residence times. The stability of chromium oxide in supercritical water was studied to determine the feasibility of using catalysts during supercritical water oxidation. Various process variables, including oxygen and water concentrations, fluid flow rate, and temperature were found to affect the reactivity of the chromium oxide catalyst. Under these conditions, chromium was present in the reactor effluent as chromic acid, H2CrO4. The visual observation of the catalyst after the completion of the experiments suggested that a reaction front moved along the length of the reactor. A rate expression, based on Langmuir–Hinshelwood kinetics, with dissociative chemisorption of oxygen on a hydrated chromium oxide surface, was proposed. The model, developed based on this rate expression and the continuity equation for the fluid phase, was consistent with the experimental results.
Solvatochromic characterization of the liquid phase in liquid—supercritical CO2 mixtures
Authors: Kelley Sean P. , Lemert Richard M.
Published: June 2004

The solvatochromic dye phenol blue (N, N‐dimethylindoaniline) is used to characterize the solvent strength (polarity) of the saturated liquid phase in a series of solvent–carbon dioxide binary mixtures. Data were obtained at 35 and 55°C and at pressures up to ∼70 bar. Five solvents were investigated—acetone, cyclohexane, methanol, THF, and toluene. The polarity of the liquid phase decreases significantly with increasing pressure due to the increasing carbon dioxide content of this phase at equilibrium. For example, the polarity of acetone saturated with carbon dioxide at 35°C and ∼60 bar is equivalent to the polarity of pure cyclohexane at ambient pressure. The local environment about the dye is significantly richer in the polar liquid component than the bulk composition would indicate. The degree of enrichment reflects concentration effects at low pressure, and both concentration and pressure effects at high pressure where the mixtures are highly compressible. The NRTL model of Renon and Prausnitz is able to predict these local compositions with reasonable accuracy except at CO2‐rich conditions where compressibility effects are important.
Modeling of solid/liquid expression for cellular materials
Authors: Lanoisellé Jean‐Louis , Vorobyov Eugène I. , Bouvier Jean‐Marie , Pair Georges
Published: June 2004

A general theory is developed for the mechanical expression of agrofood, cellular materials. The so‐called, Liquid‐containing biporous particles expression model considers liquid transfer within a network of three different volumes: extraparticle, extracellular and intracellular volumes. The system of partial differential equations is solved for the expression under constant pressure, allowing calculation of the total layer settlement, as well as the deformation of extraparticle, extracellular and intracellular volumes. The model is able to predict the behavior of four different steps in the consolidation stage: the primary deformation and the creep deformation of extraparticle volume, and the deformation and deliquoring of both extracellular and intracellular volumes. The model is applied to the hydraulic pressing of rapeseeds. The theoretical model agrees well with experimental data for the overall range of pressing time.
Effect of unstable density gradients on back‐mixing in a reciprocating plate column
Authors: Aravamudan Kannan , Baird Malcolm H. I.
Published: June 2004

Axial dispersion coefficients were measured in the continuous phase in a 5‐cm‐dia. Karr column, operated cocurrently with the system kerosene(d)‐water(c). The effect of unstable density gradient in the continuous phase was studied by injecting a steady stream of sodium chloride solution as a tracer near the top of the column. The effects of energy dissipation due to unstable density gradient, mechanical agitation, and the flow of the dispersed phase were investigated as the three principal variables, and two plate spacings (2.55 and 5.10 cm) were also employed. The axial dispersion results were correlated in terms of a weighted power‐law model based on Kolmogoroff's mixing length concept and incorporating contributions due to each of the three modes of energy dissipation. Axial mixing was significantly increased in the presence of unstable density gradients even though these contributed very little (typically less than 1 mW/kg) to the total specific energy dissipation rate.
Batch tracking via nonlinear principal component analysis
Authors: Dong Dong , McAvoy Thomas J.
Published: June 2004

Batch processes are very important to the chemical and manufacturing industries. Techniques for monitoring these batch processes to ensure their safe operation and to produce consistently high‐quality products are needed. Nomikos and MacGregor (1994) presented a multiway principal component analysis (MPCA) approach for monitoring batch processes, and test results show that the method is simple, powerful, and effective. MPCA, however, is a linear method, and most batch processes are nonlinear. Although data treatment techniques can remove some nonlinearity from the data, nonlinearity is still a problem when using MPCA for monitoring. In this article a nonlinear principal component analysis (NLPCA) method (Dong and McAvoy, 1993) is used for batch process monitoring. Results show that this method is excellent for this problem. Another interesting extension of this approach involves multistage batch process monitoring, which is illustrated through a detailed simulation study.
Mediating operation of heterogeneous CSTR
Authors: Gol'dstein Vladimir , Panfilov Vadim , Shreiber Isaac
Published: June 2004

A new type of periodic operation is investigated for a simple model of a heterogeneous catalytic continuous stirred tank reactor. For the heterogeneous case, two scales of response times were derived, slow for heat processes and fast for concentration processes. Periodic operation in intermediate mode between slow and fast processes is studied by manipulation of the feed flow rate. The new approach is designated as mediating operation, which demonstrates ability to control interaction between fast and slow reactor subsystems due to desired separation of dynamic responses for temperature and concentration processes by a single input variable. The conversion improvement at lower temperature can be achieved for lower and upper stable steady states. For the system analysis, analytical investigation of the slow manifolds is used as well as numerical study.
Compressive yield stress of flocculated suspensions: Determination via experiment
Authors: Green Matthew D. , Eberl Maria , Landman Kerry A.
Published: June 2004

The compressive yield stressPy(ϕ) is an extremely useful parameter for the characterization of concentrated flocculated suspensions. It is a measure of the strength of inter‐particle bonds in a networked suspension structure and is used in process design and control of mineral colloidal suspension dewatering. Previous work developed a method for determining this compressive yield stress via the equilibrium sediment height technique using a centrifuge. Here justification for a simple approximate technique to obtainPy(ϕ) is discussed. A new more accurate iterative algorithm is also described. By generating synthetic data, where the exactPy(ϕ) is known, the accuracy of the two techniques can be evaluated properly for the first time. The relative merits and failings of the techniques are discussed. An example is given to show how these techniques work on real experimental data.
Phase equilibria for aqueous protein/polyelectrolyte gel systems
Authors: Sassi Alexander P. , Blanch Harvey W. , Prausnitz John M.
Published: June 2004

A molecular‐thermodynamic analysis is directed toward predicting the partitioning of aqueous proteins into charged hydrogels. This analysis takes into account size exclusion by the network, electrostatic interactions, and the osmotic‐pressure difference between a hydrogel and its surrounding solution. Electrostatic interactions in the polyelectrolyte gel can be described by Debye – Hückel theory, or the Mean Spherical Approximation, or Katchalsky's cell model for polyelectrolyte solutions. The cell model gives best agreement with experimental partition coefficients for cytochrome c. The quasi‐electrostatic potential difference between a gel and its surrounding solution demonstrates how the electrostatic contribution to the protein partition coefficient depends on protein charge, gel‐charge density, and solution ionic strength. Finally, a qualitative guide is presented for design of a polyelectrolyte gel such that it exhibits specified swelling and partitioning properties.
Collisional viscosity of FCC particles in a CFB
Authors: Gidaspow Dimitri , Huilin Lu
Published: June 2004

Particle velocity distributions were measured for a flow of 75‐μm FCC particles in a CFB using a new video‐digital camera technique. From the spread of particle histograms, a random oscillating particle velocity was determined. This random velocity was used to compute the powder viscosity with the help of dense‐phase kinetic theory of granular flow. There was excellent agreement between this kinetic‐theory measurement and previous macroscopic viscosity measurements. The dimensionless group kinematic viscosity divided by the oscillating velocity and particle diameter correlated all available data as a function of particle volume fraction.
Modeling gradient elution of proteins in ion‐exchange chromatography
Authors: Gallant Stuart R. , Vunnum Suresh , Cramer Steven M.
Published: June 2004

The steric mass action (SMA) model of ion exchange is employed in concert with appropriate mass balance equations to describe the behavior of a concentrated band of protein under gradient‐elution conditions. A method of characteristics approach is employed to solve the model equations rapidly. The model is shown to accurately predict the gradient elution of the protein cytochrome C in a strong cation‐exchange column. This work constitutes an advance over previously available models as it allows engineers to rapidly consider the effects of nonlinear adsorption in gradient‐elution chromatography using a spreadsheet. Further, it provides significant physical insight into the process of gradient elution for preparative and large‐scale separations.
Enhanced automatic tuning procedure for process control of PI/PID controllers
Authors: Tan K. K. , Lee T. H. , Wang Q. G.
Published: June 2004

An 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

A 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

A 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

The 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

The 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

A 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

In 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

The 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

Even 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

Feedforwar/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

To 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

The 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

Pulsed‐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

For 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

This 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

Chemical 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.