bluuebunny/crossref_metadata_2025 huggingface dataset:
Pressure‐fluctuation analysis of a Gas–Solid fluidized bed using the wigner distribution
Authors: He Zhen , Zhang Dongming , Cheng Bochuan , Zhang Weidong
Published: June 2004

Pressure fluctuations in gas–solid fluidized beds have long been recognized as an important index for the quality of fluidization. Due to their highly random nature, many researchers have resorted to stochastic signal‐processing methods to extract useful information. However, methods such as power‐spectral density are based on a priori hypothesis of the stationarity of the signals, whose validity has not been investigated. In this article, using the Wigner distribution (WD), the signal's strong nonstationary feature is demonstrated and signal properties are addressed. Local frequency information is extracted from the defined parameter—the local peak weighted average (LPWA), which can be regarded as a generalized concept of the major frequency of power‐spectral density in the nonstationary case. Experimental results indicate that LPWA is stochastically much more reproducible than the major frequency of power‐spectral density. The change in LPWA under different operating conditions may reflect the corresponding change in the bubble phase. This information is useful in characterizing the quality of fluidization.
Comminution of limestone during batch fluidized‐bed calcination and sulfation
Authors: Scala Fabrizio , Cammarota Antonio , Chirone Riccardo , Salatino Piero
Published: June 2004

Batchwise fluidized‐bed calcination and sulfation of a limestone were done to investigate particle comminution phenomena and their relation with parallel occurrence of reactions. Operating conditions of the bed were those typical of atmospheric bubbling fluidized‐bed combustors. A general framework of comminution phenomena is outlined, which includes different types of fragmentations as well as attrition by abrasion. Comminution processes were characterized by following the modifications of bed sorbent particle‐size distribution and the elutriation rates of fines throughout conversion. Mutual interactions between comminution processes and the progress of chemical reactions are assessed.
Attainable regions for reaction with separation
Authors: Nisoli Alberto , Malone Michael F. , Doherty Michael F.
Published: June 2004

The attainable region approach for reaction – mixing systems is combined with geometric methods for the feasibility of separations. The result is a systematic approach to identify the feasible compositions that can be achieved in processes combining simultaneous reaction, mixing, and separation. An activity‐based formulation for both nonideal VLE and reaction‐rate expressions is applied to develop hybrid reactor–separator models for the multiphase CSTR and PFR with simultaneous vapor removal. A reaction–separation vector is defined that satisfies the same geometric properties as the reaction vector. Therefore, the attainable region can be constructed following the existing procedure for reaction–mixing systems. This approach provides a method to generate feasible process alternatives. The technique is demonstrated on two nonideal ternary mixtures: the production of dimethyl ether by dehydration of methanol, and the production of methyl tert‐butyl ether from isobutene and methanol. It is shown that for hybrid reaction–separation devices the entire composition space is not always attainable. In such cases, combining a hybrid device with traditional nonreactive separation is required to attain certain products.
Chemical imaging of multicomponent viscous fingering in chromatography
Authors: Dickson Matthew L. , Norton T. Tucker , Fernandez Erik J.
Published: June 2004

Resolution in chromatographic separations can be compromised by nonuniform flow distribution, heterogeneous packing, or flow instabilities such as viscous fingering. In this work, the displacement of solutes within a chromatographic column was monitored with an advanced magnetic‐resonance technique, echo‐planar spectroscopic imaging (EPSI). Unlike conventional magnetic‐resonance imaging, this approach allows multiple solutes to be observed simultaneously, providing separate “chemical images” for each component. Qualitative chemical images were obtained in approximately 1 min with approximately 1 mm3spatial resolution. EPSI allowed us to examine the multi‐component viscous fingering problem for the first time. While many features of the instability are preserved, solute retention appears to slow the growth of solvent fingers.
High‐purity Nitrogen by pressure‐swing adsorption
Authors: Shirley Arthur I. , Lemcoff Norberto O.
Published: June 2004

The separation of air for nitrogen production can be carried out by pressure‐swing adsorption over a carbon molecular sieve. The separation is kinetically based, since the equilibrium adsorption of both oxygen and nitrogen is very similar, but the oxygen is adsorbed faster. Several theoretical and experimental studies have been reported, but mainly dealing with nitrogen purities below 99.9%. The objective of this article is to study experimentally the effect of different process variables on the performance of a rate‐induced PSA process in the high‐purity region. The effect of cycle time on both the yield and the specific product per unit cycle become more significant as the purity increases. On the other hand, the specific feed per unit cycle is almost insensitive to product purity, particularly as the cycle time increases. These results agree very well with predictions from a theoretical model.
Granulation of sol‐gel‐derived nanostructured alumina
Authors: Deng S. G. , Lin Y. S.
Published: June 2004

A sol‐gel granulation process was developed to prepare porous nanostructured γ‐alumina granules as supports for catalysts and adsorbents. The process, which starts with an aqueous sol of gelatinous boehmite, involves droplet formation, gelation in paraffin oil, conditioning in ammonia solution, and drying and calcination in air under controlled conditions. The γ‐Al2O3granules prepared are 1–3 mm‐dia. spherical particles with large surface area (380 m2/g) and pore volume (0.5 cm3/g), uniform pore‐size distribution (20–60 Å), and controllable average pore size (35 Å). These sol‐gelderived granules have excellent mechanical properties with crush strength (>100 N per granule) and attrition resistance (<0.01 wt. %/h), much better than the commercial alumina and zeolite granules. Supported CuO sorbents were prepared on these granules for SO2removal applications. The alumina‐supported CuO sorbents contain higher loading of well‐dispersed CuO and better sulfation properties than similar sorbents reported in the literature.
Cosolvent tuning of tautomeric equilibrium in supercritical fluids
Authors: Dillow Angela K. , Hafner Kellye P. , Yun S. L. Jimmy , Deng Fenghua , Kazarian Sergei G. , Liotta Charles L. , Eckert Charles A.
Published: June 2004

Cosolvents offer a sensitive tool to tailor equilibria, rates or yields of chemical reactions in supercritical fluid solvents. The tautomeric equilibrium of a Schiff base was chosen as a model system, and small amounts of protic cosolvents shift the position of tautomeric equilibria. The equilibrium was tuned from essentially one tautomer to another by modifying the solvent of pure SCF ethane with less than 2 mol % hexafluoroisopropanol cosolvent. The equilibrium constant was a function of cosolvent concentration and mixture density. A chemical‐physical model of equilibrium constants deviates from the measured values in the near‐critical region, which may have been caused by local composition enhancement of cosolvent around the Schiff base. Decrease in the degree of hydrogen bonding with pressure or density affects the keto‐enol equilibria by decreasing the amount of the keto formation. Thus, solution density, as well as the degree of hydrogen bonding, is manipulated to tune the position of tautomeric equilibria.
Effluent concentration profiles in centrifugal partition chromatography
Authors: van Buel M. J. , van der Wielen L. A. M. , Luyben K. Ch. A. M.
Published: June 2004

Centrifugal partition chromatography (CPC) is a relatively new preparative chromatographic technique. To understand, predict, and optimize CPC separations a model is needed, describing the effluent concentration profile as a function of the phenomena that determine the separation efficiency (mass transfer, mixing, and partitioning). The model presented in this article describes experimental effluent concentration profiles accurately. Partition coefficients, Stanton numbers, and Péclet numbers were obtained by comparing model simulations to experimental pulse‐response data. The fitted partition coefficients agree well with those obtained from shake‐flask experiments. Mass‐transfer limitation is the major reason for peak broadening. The inverse mass‐transfer coefficient is a linear function of the partition coefficient. The model will be a valuable tool in determining the influence of mass transfer as a function of various experimental conditions.
Nonlinear control of a high‐purity distillation column using a traveling‐wave model
Authors: Balasubramhanya Lalitha S. , Doyle Francis J.
Published: June 2004

The trade‐off between model accuracy and computation tractability for model‐based control applications is well known. While nonlinear models are needed to capture the detailed behavior of many chemical processes, the resultant structures may not lead to straightforward control implementation. The current work advocates the use of low‐order nonlinear models based on wave propagation that are mathematically concise and capture the essential nonlinear behavior of a process. In this work, differential geometry is invoked to achieve input–output linearization of a high‐purity distillation column based on a traveling‐wave model. A Kalman filter is used to recursively update the parameter values. Comparison with a linear controller based on a two‐time constant model shows that the nonlinear controller outperforms the linear controller in tight control of both the overhead and the bottoms composition.
Modeling and optimization of a semiregenerative catalytic naphtha reformer
Authors: Taskar Unmesh , Riggs James B.
Published: June 2004

Modeling and optimization of a semiregenerative catalytic naphtha reformer has been carried out considering most of its key constituent units. A detailed kinetic scheme involving 35 pseudocomponents connected by a network of 36 reactions in the C5–C10 range was modeled using Hougen‐Watson Langmuir‐Hinshelwood‐type reaction‐rate expressions. Deactivation of the catalyst was modeled by including the corresponding equations for coking kinetics. The overall kinetic model was parameterized by bench‐marking against industrial plant data using a feed‐characterization procedure developed to infer the composition of the chemical species in the feed and reformate from their measured ASTM distillation data. For the initial optimization studies, a constant reactor inlet temperature configuration that would lead to optimum operation over the entire catalyst life cycle was identified. The analysis was extended to study the time‐optimal control profiles of decision variables over the run length. In addition, the constant octane case was also studied. The improvement in the objective function achieved in each case was determined. Finally, the sensitivity of the optimal results to uncertainty in reactor‐model parameters was evaluated.
Microwave heating of the continuous flow catalytic reactor in a nonuniform electric field
Authors: Plazl I. , Pipus G. , Koloini T.
Published: June 2004

Temperature profiles in the continuous‐flow reactor with a fixed bed placed in a nonuniform microwave field are predicted by considering specific on – off regulation of the microwave power and local distribution of the electric‐field intensity. The spatial and temperature dependence of the electric‐field intensity affects the heating appreciably, as shown by indirect measurements. Results of numerical calculations agree with experimental data of outlet temperature and explain temperature oscillations in the outlet flow. The model developed allows an accurate description of microwave heating in the continuous‐flow reactor associated with specific regulation of microwave power. The hydrolysis of sucrose catalyzed by the strongly acidic cation‐exchange resin Amberlite 200C in R‐H form was chosen as the reaction system. Calculated conversions of sucrose based on predicted temperature profiles agree with experimental data.
Degradation kinetics of polymers in solution: Dynamics of molecular weight distributions
Authors: McCoy Benjamin J. , Madras Giridhar
Published: June 2004

Polymer degradation occurs when macromolecular chains are broken under the influence of thermal, mechanical or chemical energy. Chain‐end depolymerization and random‐ and midpoint‐chain scission are mechanisms that have been observed in liquid‐phase polymer degradation. Here we develop mathematical models, unified by continuous‐mixture kinetics, to show how these different mechanisms affect polymer degradation in solution. Rate expressions for the fragmentation of molecular‐weight distributions (MWDs) govern the evolution of MWDs. The governing integrodifferential equations can be solved analytically for realistic conditions. Moment analysis for first‐order continuous kinetics shows the temporal behavior of MWDs. Chain‐end depolymerization yields monomer product and polymer molecular‐weight moments that vary linearly with time. In contrast, random‐ and midpoint‐chain scission models display exponential time behavior. The mathematical results reasonably describe experimental observations for polymer degradation. This approach, based on the time evolution of continuous distributions of chain length or molecular weight, provides a framework for interpreting several types of macromolecular degradation processes, particularly how bimodal MWDs can evolve during degradation.
Crystal unfolding and chain disentanglement during semicrystalline polymer dissolution
Authors: Mallapragada Surya K. , Peppas Nicholas A.
Published: June 2004

Mathematical models developed describe the unfolding of polymeric crystals in the presence of a solvent followed by their subsequent disentanglement. A thermodynamic model considering the free energy changes during crystal unfolding was proposed to obtain an expression for the unfolding rate. A simplified version of this expression, assuming uniform crystal size, was incorporated into a continuum model to predict the dissolution kinetics of a semicrystalline polymer slab. The model yielded predictions of the crystalline and solvent volume fractions as a function of position within the slab, in addition to changes in the fraction of polymer dissolved as well as the degree of crystallinity of the polymer as a function of time. The degree of crystallinity of the polymer decreased with time and the plot of the fraction of the polymer dissolved as a function of time exhibited Case II behavior. The model predictions agreed will with experimental results obtained during dissolution of semicrystalline poly(vinyl alcohol) in water.
Unifying approach to modeling granule coalescence mechanisms
Authors: Adetayo A. A. , Ennis B. J.
Published: June 2004

A novel, physically based kernel for population balance modeling of granule growth by coalescence is presented. This kernel is size‐independent in that all collisions with an effective average granule size less than a critical value are successful. Simulations based on this kernel show that a variety of contradictory experimental observations can be modeled. In the limiting case of viscoelastic collisions, the kernel can be related to the governing group of the Stokes number (Ennis et al., 1991), representing the ratio of granule collisional kinetic energy to viscous dissipation brought about by the binder. In more general cases, material properties that control deformability, such as interparticle friction, binder viscosity, and liquid content, strongly affect this critical size. The kernel clearly demonstrates the three regimes of drum granulation originally proposed by Kapur and Fuerstenau in 1964 and compares favorably with the two‐stage sequential kernel developed by Adetayo et al. in 1995 for the drum granulation of fertilizers.
Multivessel batch distillation
Authors: Skogestad Sigurd , Wittgens Bernd , Litto Rajab , Sørensen Eva
Published: June 2004

The multivessel batch column consists of a reboiler, several column sections, intermediate vessels and a condenser vessel. This configuration provides a generalization of previously proposed batch‐distillation schemes, including the inverted column and the middle‐vessel column. The total reflux operation of the multivessel batch‐distillation column was presented recently, and the main contribution of this article is to propose a simple feedback control strategy for its operation. We propose to adjust the vessel holdups indirectly by manipulating the reflux flow out of each vessel to control the temperature at some location in the column section below. The feasibility of this strategy is demonstrated by simulations.
Parallel frontal solver for large‐scale process simulation and optimization
Authors: Mallaya J. U. , Zitney S. E. , Choudhary S. , Stadtherr M. A.
Published: June 2004

For the simulation and optimization of large‐scale chemical processes, the overall computing time is often dominated by the time needed to solve a large sparse system of linear equations. A new parallel frontal solver that can significantly reduce the wallclock time required to solve these linear equation systems using parallel/vector supercomputers is presented. The algorithm exploits both multiprocessing and vector processing by using a multilevel approach in which frontal elimination is used for the partial factorization of each front. Results of several large‐scale process simulation and optimization problems are presented.
New simplified models and formulations for turbulent flow and convection
Authors: Churchill Stuart W.
Published: June 2004

Expression of the time‐averaged equations for the conservation of momentum and energy in terms of the fractions of the local shear stress and heat flux density, respectively, due to turbulence is shown to result in greatly simplified representations for fully developed flow and convection in round tubes and parallel‐plate channels as compared to those in terms of the eddy viscosity, eddy conductivity, or mixing length. On the other hand, the turbulent Prandtl number is shown to be a fundamental characteristic of flow and convection rather than simply an artifact of the eddy diffusional model. The new simplified representations provide the basis for improved predictions of the velocity distribution, the friction factor, the temperature distribution, and the Nusselt number. The adaption of these new representations for other geometries and for developing convection is discussed briefly.
Wavelet‐based density estimation and application to process monitoring
Authors: Safavi A. A. , Chen J. , Romagnoli J. A.
Published: June 2004

An application of wavelets and multiresolution analysis to density estimation and process monitoring is presented. Wavelet‐based density‐estimation techniques are developed as an alternative and superior method to other common density‐estimation techniques. Also shown is the effectiveness of wavelet estimators when the observations are dependent. The resulting density estimators are then used in defining a normal operating region for the process under study so that any abnormal behavior by the process can be monitored. Results of applying these techniques to a typical multivariate chemical process are also presented.
Circulation in bubble columns
Authors: Burns L. F. , Rice R. G.
Published: June 2004

Liquid circulation in bubble columns influences many system properties such as mass transfer and catalyst dispersion. Conditions favorable for liquid circulation often exist only for highly turbulent slurries. Experiments under conditions of lowered surface tension proved conclusively that uniform circulation can exist even in the bubbly‐flow regime. Moreover, the measured circulating velocity profile is more plug‐shaped in the center core than traditional models, which take a parabolic shape. These observations led to the application of a circulation theory based on an energy‐dissipation model for turbulent eddy viscosity, which can be easily computed as a simple function of superficial‐gas velocity. Two length scales were used in model calculations depending on flow regime: bubble diameter for bubbly flow and column diameter for churn turbulence. The liquid‐velocity profile contained a tuned dimensionless proportionality constant, which resulted in two different average values, k = 2.51 for bubbly flow and 0.0242 for churn turbulence, when tuned to experimental data. The integral average eddy viscosity is numerically equivalent to the eddy dispersion coefficient for bubbly flow. Predicted liquid velocity compared very favorably with new measured values under conditions of low surface tension and with literature values.
Turbulence structure and prediction of interfacial heat and mass transfer in wavy‐stratified flow
Authors: Lorencez C. , Nasr‐Esfahany M. , Kawaji M.
Published: June 2004

The turbulence structure of the liquid phase near a wavy gas‐liquid interface in stratified flow was experimentally investigated in a 50 mm (H) × 100 mm (W) rectangular duct. The characteristic parameters of the organized motion under the liquid waves such as the frequency of appearance and length scale were estimated by using the variable‐interval time‐averaging and the photochromic dye activation techniques. These characteristics were used in a hybrid surface renewal‐eddy cell model to predict the interfacial heat‐ and mass‐transfer coefficients in stratified two‐phase flow. The predictions for cocurrent and countercurrent flows agreed reasonably well with experimental data available in the literature.
Evaluation of gas‐solids mixing chamber through cross correlation and Hurst's analysis
Authors: Briens C. L. , Mirgain C. , Bergougnou M. A. , Del Pozo M. , Loutaty R.
Published: June 2004

Fast circulating fluidized‐bed reactors are used for many applications, both in riser and downer modes. Typical resisdence times are in the order of a few hundreds of milliseconds. A good understanding of initial gas‐solids contact must be reached to realize the full potential of such reactors. Effective and stable gas‐solids mixing chambers were identified from the singals of simple and robust probes, which could be used in industrial and pilot plants. Criteria based on cross correlation or Hurst analysis provided consistent results. Although this study used simplified momentum probes, the criteria it developed can be applied to the signal of any fast‐response sensor which is affected by local hydrodynamics. Temperature, capacitance, ultrasonic, or γ‐ray absorption measurements could be used. Gas or liquid‐liquid mixing chambers could also be optimized with the same criteria.
NMR measurements and numerical simulation of fluid transport in porous solids
Authors: Tessier J. J. , Packer K. J. , Thovert J.‐F. , Adler P. M.
Published: June 2004

Pulsed magnetic field gradient stimulated echo NMR measurements are performed for diffusion and flow of an aqueous phase both within a sample of packed spherical beads and within a 25% porosity Fontainebleau sandstone. The stimulated echo dependence on the gradient pulse area q is used to derive the displacement probability distributions PΔ(X) for fixed observation times Δ. The shape of PΔ(X) as a function of Δ is simulated for computer‐generated porous media, and a good agreement is obtained between the experimental NMR data and the simulations.
Reaction rates in fractal vs. uniform catalysts with linear and nonlinear kinetics
Authors: Gavrilov C. , Sheintuch M.
Published: June 2004

Pore fractal objects are expected to be optimal catalysts, since material is supplied to the narrower pores, which are also shorter through the larger pores where diffusion resistance is smaller. To demonstrate this, diffusion and reaction were simulated on Sierpinski‐gasket‐type fractal objects and on the corresponding nonfractal uniform‐pore structures of the same size, porosity and reactive area. Positive order reactions limited by Knudsen diffusion were shown to exhibit larger rates in fractal than in uniform‐pore objects. Fractal catalysts also exhibited a new intermediate domain in which the rate depends only weakly on the kinetics parameters. In nonmonotomic kinetics the branching point (bifurcation point) was extremely sensitive to the pore structure.
Use of a novel autoassociative neural network for nonlinear steady‐state data reconciliation
Authors: Du Yang‐Guang , Hodouin Daniel , Thibault Jules
Published: June 2004

A novel autoassociative neural‐network‐based estimator for nonlinear steady‐state data reconciliation was developed, which is a modified autoassociative feedforward neural network. The main difference between them lies in the minimization of an objective function that includes material imbalance terms of flow rates and compositions as well as the traditional least‐square prediction term. Accordingly, this neural network, with the material balance‐related equations included in the objective criterion, can perform simultaneously the following basic functions necessary for proper steady‐state data rectification: (1) eliminate the nonrandom errors, such as the biases and gross erros, from measurements; (2) filter out the random errors from measured data; and (3) estimate the values of unmeasured process variables, provided data redundancy prevails. This novel neural‐network‐based reconciliation method is demonstrated on a flotation system.
Dehydrogenation of propane using a packed‐bed catalytic membrane reactor
Authors: Weyten Herman , Keizer Klaas , Kinoo Annemieke , Luyten Jan , Leysen Roger
Published: June 2004

In a high‐temperature catalytic membrane reactor, a plug‐flow reactor is combined with a gas‐separative membrane. By selectively removing one of the reaction products, the reaction mixture is prevented from reaching equilibrium, and a higher conversion can be obtained. This concept is only valid for reactions that are limited by the thermodynamic equilibrium, such as the direct dehydrogenation of propane to propene. A tubular H2‐selective silica membrane was characterized [αH2/C3H8 = 70–90 at 500°C] and used as the gas‐separative membrane. The membrane reactor was filled with a chromia/alumina catalyst. The kinetics of the catalyst was studied. At 500°C the deactivation of the catalyst is slow, and the propene yield is almost constant for at least 10 h of operation. Under well‐chosen process conditions, the propene yield is at least twice as high as the value obtained at thermodynamic equilibrium in a conventional reactor.
Polymer trace devolatilization: I. Foaming experiments and model development
Authors: Yang Chi‐Tai , Smith Theodore G. , Bigio David I. , Anolick Colin
Published: June 2004

Volatile species are removed from polymers by devolatilization. Foaming experiments using corn syrup and low‐density polyethylene were carried out to investigate effects of volatile content, operating vacuum level and stripping agent in devolatilization. When foaming and rupture occur, a limiting foam volume growth was observed for a polymer regardless of the volatile content and the addition of stripper bubbles. A mathematical model was also developed to study the effect of stripper bubbles in polymer devolatilization. Based on the cell model for bubble growth, a bimodal model for foam growth is developed as the first step to describe the initial formation and growth of the volatile and stripper bubbles in polymer during devolatilization. When foaming and breakup occur at a critical foam volume expansion, a film model for mass transfer is used to model the second step of devolatilization. A numerical factorial study shows that the devolatilization section should be designed and operated to provide the screw channel sufficient free volume and residence time for the desired foam expansion.
Improving clay‐based tailings disposal: Case study on coal tailings
Authors: de Kretser Ross , Scales Peter J. , Boger David V.
Published: June 2004

The role of swelling clays in hindering the compressional dewatering characteristics of coal‐mine tailings is examined. The effects of electrolyte concentration and ion exchange in improving the shear and compressional rheology are compared. Suspensions studies include actual mine tailings (thickener feed and thickener underflow) as well as synthetic clay dispersions made from clay collected from the coal seam. It was shown that the most important parameter in controlling the properties of the tailings suspension is “controlled” dispersion in the presence of a Ca2+ electrolyte concentration in excess of that required to (1) prevent initial swelling and (2) provide full cation exchange of the clay. Under these electrolyte conditions, complete delamination of the clay did not occur, and both the dewatering and handling characteristics of the resultant suspensions improved dramatically.
Extracting effective diffusion parameters from drying experiments
Authors: Price Peter E. , Wang Sharon , Romdhane Ilyess Hadj
Published: June 2004

Thin polymer coatings produced from water‐ and organic‐solvent‐based precursor liquids are ubiquitous components of industrial and consumer products. Design and optimization of dryers for these coatings require accurate predictions of their drying rates. Accurate drying predictions for many polymer/solvent coatings require a satisfactory description of the polymer/solvent mutual diffusion coefficients. A method is proposed ot determine a satisfactory description of the diffusion coefficients by effective free‐volume theory parameters. The effective parameters are determined from gravimetric data measured in a bench‐scale drying apparatus. The application of the method to a poly(vinyl acetate)/toluene solution shows that the effective free‐volume parameters give diffusion coefficients that agree with published data. The method is also applied to a rubber‐based adhesive in industrial‐grade heptane by treating the complex adhesive as a pseudobinary solution. For both systems, the effective parameters lead to quantitatively accurate drying predictions in a pilot‐scale oven.
Control‐oriented modeling of sheet and film processes
Authors: Featherstone Andrew P. , Braatz Richard D.
Published: June 2004

Sheet and film processes pose a challenging identification and control problem due to high complexity, poor conditioning, and limited input–output data. The interaction between model accuracy and closed‐loop performance is explored for sheet and film processes using a model decomposition in terms of two static orthogonal matrices in series with a diagonal transfer‐function matrix. It is shown that the accuracy of the diagonal elements of the transfer‐function matrix directly specifies the closed‐loop performance achievable by a model‐based controller. This motivates the development of a combined identification and control procedure in which the controller is designed to be robust to model inaccuracies quantified during identification. The resulting controller is compared to an industrially accepted controller design method for two examples, including a simulated blown‐film process. Based on the theoretical results and simulations, it was concluded that the poor performance often reported for existing industrial sheet and film process‐control systems is likely due to signs of the model gains being incorrectly identified.
Design analysis for refolding monomeric protein
Authors: Kotlarski Nicholas , O'Neill Brian K. , Francis Geoffrey L. , Middelberg Anton P. J.
Published: June 2004

Renaturation of protein expressed as inclusion bodies within Escherichia coli is a key step in many bioprocesses. Operating conditions for the refolding step dramatically affect the amount of protein product recoverd, and hence profoundly influence the process economics. The first systematic comparison of refolding conducted in batch, fed‐batch and continuous stirred‐tank reactors is provided. Refolding is modeled as kinetic competition between first‐order refolding (equilibrium reaction) and irreversible aggregation (second‐order). Simulations presented allow direct comparison between different flowsheets and refolding schemes using a dimensionless economic objective. As expected from examination of the reaction kinetics, batch operation is the most inefficient mode. For the base process considered, the overall cost of fed‐batch and continuous refolding is virtually identical (less than half than of the batch process). Reactor selection and optimization of refolding using overall economics are demonstrated to be vitally important.
Recovering deterministic behavior from experimental time series in mixing reactor
Authors: Letellier C. , Sceller L. Le , Gouesbet G. , Lusseyran F. , Kemoun A. , Izrar B.
Published: June 2004

The velocity field in a standard mixing reactor with a Rushton impeller is analyzed by using techniques from the theory of nonlinear dynamical systems. It is shown that the dynamical behavior contains a quasi‐periodic motion with three frequencies, fp, the frequency associated with the rotation of blades, fp/6, and a third frequency f'. Relying on an evaluation of the correlation dimension equal to 3.9, the phase space is likely to be at least four‐dimensional. Moreover, a set of four ordinary differential equations is indeed automatically obtained by using a global vector field reconstruction technique, confirming the existence of a 4–D‐deterministic behavior contributing to the dynamics of the system.
Temperature dependence of one‐component permeation through a silicalite‐1 membrane
Authors: Bakker Wridzer J. W. , Van Den Broeke Leo J. P. , Kapteijn Freek , Moulijn Jacob A.
Published: June 2004

The one‐component steady‐state permeation of gases through a silicalite‐1 zeolite composite membrane as a function of the temperature is studied from 190 to 680 K for light hydrocarbons, noble gases, and some inorganic gases. In general, with increasing temperature the permeance shows a maximum followed by a minimum. For gases weakly adsorbed the permeance has only a minimum and for gases strongly adsorbed only a maximum is observed in the permeance. The permeance for various gases, for a feed pressure of 101 kPa, span four orders of magnitude. The lowest permeation is for i‐butane at 300 K: a permeance of 0.07 × 10−8 mol. m−2.s−1.Pa−1. The highest value is observed for methane: a permeance of 70 × 10−8 mol. m−2.s−1.Pa−1 at about 240 K. A comparison between the isobars and the temperature dependence of the steady‐state permeance, both at 101 kPa, shows that at the temperature where the amount adsorbed vanishes the permeance starts to increase. The temperature dependence of the steady‐state fluxes through the silicalite‐1 membrane can be described only if two diffusion mechanisms are taken into account. For high occupancies the mass transport can be described by equilibrium adsorption followed by surface diffusion and for low occupancies the mass transport can be described by activated gaseous diffusion. With increasing temperature the mass‐transport mechanism shifts from the surface diffusion regime to the activated gaseous diffusion regime. With these two diffusivities modeling results agree well with experimental results for the one‐component flux through the silicalite‐1 zeolite membrane.
Constrained MPCI: A weak persistent excitation approach
Authors: Vuthandam Premkiran , Nikolaou Michael
Published: June 2004

Model‐predictive control and identification (MPCI) introduced by Genceli and Nikolaou in 1996 is a novel approach to the identification of processes under constrained model‐predictive control. MPCI solves an on‐line optimization problem that involves (a) a standard MPC objective; (b) standard MPC constraints; and (c) persistent excitation (PE) constraints. The on‐line optimization problem is computationally demanding. To alleviate that problem, we take a frequency‐domain approach to formulating the PE constraints. This approach relies on the following fact: a signal is persistently exciting of order n, if its two‐sided power spectrum is nonzero at no fewer than n points. Therefore, persistently exciting input signals can be parametrized over a finite horizon as a sum of sinusoid terms with nonzero coefficients. Used in the MPCI framework, the last requirement generates a set of completely decoupled reverse‐convex constraints that are combinatorially tractable from a computational point of view. The effectiveness of the proposed MPCI method is demonstrated through simulations. For the SISO systems studied, computation of the global optimum could be handled combinatorially in real‐time using PC hardware.
Design and retrofit of sensor networks in process plants
Authors: Bagajewicz Miguel J.
Published: June 2004

A novel procedure to find cost‐optimal sensor networks is proposed. Cost is minimized subject to qualifying constraints that are related to certain requirements of data reconciliation. One basic qualifying constraint is a desired level of precision of reconciled values for a selected set of variables. Since precision requirements lead to multiple solutions, other qualifying constraints are proposed. These constraints are availability, resilience, and error detectability. Definitions for these terms are given and their impact on the results is presented.
Pore‐structure optimization of calcium carbonate for enhanced sulfation
Authors: Mahuli Suhas K. , Agnihotri Rajeev , Chauk Shriniwas , Ghosh‐Dastidar Abhijit , Wei S.‐H. , Fan Liang‐Shih
Published: June 2004

A modified CaCO3 sorbent with an open internal pore structure is prepared and its sulfation characteristics are investigated in an entrained flow reactor at high temperatures (900‐1,100°C) and short contact times (20–600 ms) using small particle sizes (< 5 μm). The most distinguishing feature of this modified carbonate (MC) is its 70–75% sulfation conversion within 0.5 s, which is substantially higher than any other sorbents published. The MC is prepared by carbonation‐precipitation from a calcium hydroxide suspension by optimizing the operating parameters to generate carbonate particles of the desired pore structural properties. The high initial surface area combined with its open pore structure and pore‐size distribution of its calcine contribute to its high reactivity. The calcined MC possesses a significant portion of its pore volume in the 50‐200 Å range. This size range represents an optimum pore size for sulfation since it provides a reasonably high surface area and is less susceptible than < 50 Å pore sizes, to pore filling, or pore‐mouth plugging due to the formation of higher molar volume CaSO4. Investigation with other carbonates reveals that a much higher portion of their calcines' porosity lies in the smaller pores, which leads to premature termination of sulfation. Results show the impact of internal pore structure on initial reactivity and ultimate sorbent conversion.
Collection efficiency model based on boundary‐layer characteristics for cyclones
Authors: Kim W. S. , Lee J. W.
Published: June 2004

In cyclones, the boundary layer formed on the collecting‐wall surface acts as a barrier for particle migration toward the wall due to a decreased centrifugal force on particles inside the boundary layer. A new theory for high‐efficiency cyclones based on the boundary‐layer characteristics is presented. The cyclone was divided into two regions: the turbulent‐core region where the centrifugal force is large, and the near‐wall region where the centrifugal force is small. Particle trajectories in the turbulent‐core region are calculated from the mean fluid motion based on the quasi‐steady drag assumption, and the collection probability of particles in the near‐wall region is calculated by the deposition velocity that results from both turbulent diffusion and centrifugal force. The deposition velocity by centrifugal force was assumed equal to the equilibrium migration velocity at a certain point inside the boundary layer, and the distance to that point from the wall is assumed to be linearly proportional to the dimensionless‐particle relaxation time. When the proportional constant was determined by fitting the theoretical results to experimental data, the theory showed an excellent enhancement in predicting the variation of collection efficiency with the inlet flow velocity and particle size.
Isotope exchange technique for measurement of gas adsorption equilibria and kinetics
Authors: Rynders R. M. , Rao M. B. , Sircar S.
Published: June 2004

The isotope exchange technique (IET) can be used to simultaneously measure multicomponent gas adsorption equilibria and self‐diffusivities of the components in a single isothermal experiment without disturbing the overall adsorbed phase. An experimental protocol for the IET and corresponding data analysis procedures is described. Isotherms and self‐diffusivities for adsorption of N2 as a pure gas were measured on commercial samples of a carbon molecular sieve and a 4‐Å zeolite using IET, as well as those of O2 and N2 from their binary mixtures. The carbon molecular sieve did not exhibit thermo‐dynamic selectivity for air separation, but had a kinetic selectivity of O2 over N2 Mass‐transfer resistances for self‐diffusion of N2 and O2 on the carbon molecular sieve were controlled by pore mouth restrictions in the carbon, but those for adsorption of N2 into the 4‐Å zeolite by Fickian diffusion inside the adsorbent. A linear driving force model described the uptakes of N2 and O2 in the carbon molecular sieve. The Fickian diffusion model described the N2 uptake in the 4‐Å zeolite. Mass‐transfer coefficients for both O2 and N2 on the carbon molecular sieve increased linearly with increasing gas‐phase partial pressure of these gases, and the pressure of O2 did not affect mass‐transfer coefficients for N2. The self‐diffusivity of N2 in the 4‐Å zeolite decreased with increasing adsorbate loading.
Equilibrium and kinetics ofn– hexane sorption in pellets of 5A zeolite
Authors: Silva José A. C. , Rodrigues Alírio E.
Published: June 2004

Equilibrium, diffusion, and fixed‐bed adsorption of n‐hexane in pellets of 5A zeolite (Rhone‐Poulenc) were studied between 373 and 573 K and at partial pressures up to 0.2 bar. Equilibrium data were obtained in a flow microbalance system. Diffusivity was studied simultaneously by gravimetric and ZLC techniques. Adsorption isotherms were satisfactorily described by a localized adsorption model developed by Nitta et al. (1984) with only one temperature‐dependent parameter (coefficient of Henry's law). The isosteric heat of adsorption is 14.2 kcal/mol.Kinetic data clearly show that macropore diffusion is the controlling mass‐transfer mechanism. Between 473 and 573 K, pore diffusivities range from 0.11 to 0.13 cm2/s in the system He‐nC6, and from 0.06 to 0.08 cm2/s in the system N2C6. The effect of partial pressure of sorbate, temperature, and total flow rate in the behavior of the fixed‐bed unit is shown. A simple mathematical model with equilibrium and diffusivity parameters obtained in this work predicts with good accuracy all fixed‐bed adsorption experiments.
Batch chemical process quality control applied to curing of composite materials
Authors: Thomas Matthew M. , Joseph Babu , Kardos John L.
Published: June 2004

Model predictive control is used extensively to control continuous‐process systems. Application of a shrinking horizon model predictive control strategy is used to predict and control the end product quality of composite laminates produced by batch autoclave curing. The main contribution is the demonstration, using a laboratory‐scale autoclave, of the feasibility and advantages of a control strategy that adjusts the batch recipe on‐line to correct for unmeasured disturbance: entering the process. Readily available, on‐line secondary measurements are used in conjunction with the process model to predict (and hence control) quality‐related end product properties. On‐line monitoring is also used to monitor the process even after the possibility of on‐line correction has passed. The results show that this approach significantly reduces end product quality variance.
Novel swirl‐flow reactor for kinetic studies of semiconductor photocatalysis
Authors: Ray Ajay K. , Beenackers Antonie A. C. M.
Published: June 2004

A new two‐phase swirl‐flow monolithic‐type reactor was designed to study the kinetics of heterogeneous photocatalytic processes on immobilized semiconductor catalysts. True kinetic rate constants for destruction of a textile dye were measured as a function of wavelength of light intensity and angle of incidence, catalyst layer thickness, and the effect of absorption of light by liquid film on the overall rate of photocatalytic degradation. Photocatalytic activities of two commercially available TiO2 catalysts (Degussa P25 and Hombikat UV 100) were also compared for different light intensities and catalyst layer thickness. Residence time distribution and mass‐transfer limitations were evaluated. This new reactor appears to be an attractive choice for kinetic studies of heterogeneous photocatalysis.
Microwave drying of softwood in an oversized waveguide: Theory and experiment
Authors: Perré Patrick , Turner Ian W.
Published: June 2004

A comparison between experimental and theoretical results for the combined microwave and convective drying of softwood is presented. The microwave applicator used for the experiments was an oversized waveguide, and the results for both sapwood and heartwood were analyzed. To elucidate on the physics of the process at a fundamental level, a complete model is proposed, which considers the intricate link that transpires between the heat‐and mass‐transfer phenomena and the power distribution throughout the sample during drying. The resulting model, which uses a comprehensive 2‐D set of equations to describe the drying process, together with a complete 3‐D solution of the Maxwell equations within the waveguide in the time domain, can be used to investigate many aspects of dielectric drying. This research deals with the spatial variation of the power density within the material at various drying times and the effect of the anisotropy of the transfer properties on the shape and evolution of the power distribution. Most importantly, it focuses on the prediction of the location of hot spots and thermal runaway within the sample from the viewpoint of product quality. Strengths and weaknesses of the model are highlighted.
Transport analysis of diffusion‐induced bubble growth and collapse in viscous liquids
Authors: Venerus David C. , Yala Nadia
Published: June 2004

Transport models of diffusion‐induced bubble growth and collapse in viscous liquids are developed and evaluated. A rigorous model for these important phenomena is formulated taking into account convective and diffusive mass transport, surface tension, and viscous and inertial effects. Predictions for bubble growth dynamics demonstrate the importance of viscous effects in the liquid surrounding the bubble for a wide range of conditions having relevance to polymer processing operations. It is also shown how previous models employing various approximations can be derived from the rigorous model by using different mathematical methods and dimensional analysis. Predicted bubble growth dynamics from the rigorous and approximate models are compared to establish the ranges of validity for two commonly used approximations. These comparisons indicate that models using a thin boundary‐layer approximation have a rather limited range of applicability. A new approximate model, based on a previously published result, gives reasonably accurate bubble growth rate predictions with a significant reduction in computational effort.