bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of exit geometry in circulating fluidized‐bed risers
Authors: Harris A. T. , Davidson J. F. , Thorpe R. B.
Published: April 2004

The influence of exit geometry on the riser axial pressure profile in circulating fluidized beds (CFB) is examined. Experiments were performed in an academic‐scale CFB operating at conditions chosen to give dimensional similarity with an industrial CFB combustor. The influence of riser exit geometry was investigated to compare smooth and abrupt exits in terms of three different measures: (1) a dimensionless length of influence, Ω, of the exit along the riser, where Ω = (length of influence)/(riser height); (2) the exit reflection coefficient, Rf, defined by Senior in 1992 where Rf = (downward solids flow in riser)/(upward solids flow in riser) just below the riser exit; and (3) the increase in cross‐sectionally averaged solids concentration at the riser exit. Analysis of the new data and results from the literature, lead to the description of solids flow at the exit in terms of a riser exit Froude number (FrR) where FrR = ust/gR. A correlation between the riser exit Froude number and the exit reflection coefficient is presented. A serious correlation between the dimensionless length of influence of the exit bend, measured down the riser, and operating conditions, particle properties, gas properties and riser dimensions is also presented. Both correlations predict the influence of exit geometry (the exit effect) on the riser axial pressure profile. The relationship between operating parameters and the increase in cross‐sectional averaged solids concentration at the riser exit is also examined.
Evolutionary operation and control of chromatographic processes
Authors: Nagrath Deepak , Bequette B. Wayne , Cramer S. M.
Published: April 2004

A novel generalized run‐to‐run control (GR2R) control strategy is presented for the optimization and control of nonlinear preparative chromatographic processes. The GR2R approach synergistically employs a hybrid (both physical and empirical) model to control chromatographic processes in the presence of sporadic and autocorrelated disturbances. First, parameters of the physical model through experiments are determined, and then the physical model is used to estimate initial parameters of the nonlinear empirical model (Hammerstein) using orthogonal forward regression. Parameters of the nonlinear empirical model are updated at the end of each run using a nonlinear recursive parameter estimation method. The updated empirical model is then used in the control algorithm (model predictive control) to estimate operating conditions for the next batch. Processes operating under fixed optimal conditions are compared with those operating with GR2R control for both gradient and displacement chromatography. The GR2R outperforms the fixed conditions in the presence of various disturbances (such as bed capacity, column efficiency, and feed load) and is an effective strategy for the optimization and control of complex chromatographic processes.
Integrated framework for abnormal event management and process hazards analysis
Authors: Dash Sourabh , Venkatasubramanian Venkat
Published: April 2004

Process hazards analysis (PHA) and abnormal event management (AEM) are important to plant safety in chemical process industries. PHA deals with the off‐line identification, assessment and mitigation of hazards, while AEM addresses process plant malfunctions on‐line. Their inherent objectives, however, are similar, such as identifying, avoiding and mitigating hazards, and planning for emergencies. While PHA reasons from causes to consequences, AEM identifies the causes from observed symptoms or faults. The PHA results contain valuable cause and consequence information, safeguards, and other operability issues. AEM can benefit from utilizing this existing knowledge about the plant. However, in current industrial practice, PHA results are not used by operators for AEM purposes. An integrated framework combining both these tasks in a synergistic manner effectively manages and displays information searched from a possibly large number of PHA results, during on‐line operation, by using a hierarchical representation of the plant. An automated methodology is developed for this representation based on topology and functional classification of equipment. The application of the integrated framework to an industrial case study is described.
Reverse‐flow operation for application of imperfectly immobilized catalysts
Authors: Hung Kim G. W. , Papadias Dennis , Björnbom Pehr , Anderlund Magnus , Åkermark Björn
Published: April 2004

A reverse‐flow packed‐bed reactor used to solve the problem of leaching from immobilized homogeneous catalysts was studied. Product‐to‐catalyst ratios (PCRs) for a reverse‐flow reactor (RFR) and a conventional batch reactor with catalyst reuse were compared theoretically and numerically. In the ideal case without band broadening in the RFR it is possible to completely eliminate the limit on PCR due to leaching, but in practice, band broadening, caused by kinetic and dispersion effects, cannot be avoided. Nonetheless, numerical experiments showed numerous cases with much higher PCRs for the RFR than for batch operation with catalyst reuse. Therefore, an RFR may be a successful solution in cases when catalyst leaching is too high for using batch operation with catalyst reuse. The design and operation of a suitable RFR are also discussed.
Effect of hydrodynamics on reactive mixing in laminar flows
Authors: Szalai E. S. , Kukura J. , Arratia P. E. , Muzzio F. J.
Published: April 2004

The effects of diffusion and convection on the evolution of reactions in chaotic flows are examined. Direct numerical simulations are applied to compute the evolution of the mixing structure, describe the stretching field, and solve the convection‐reaction‐diffusion material balance in a periodic, 2‐D chaotic flow with an instantaneous, bimolecular reaction: A + B → 2P. These computations demonstrate that the location of the reactive zones in the flow can be predicted from the stretching field. The time‐evolution of the concentration profiles approaches an invariant shape when the time scale of the convective and diffusive mixing processes are comparable. Under such conditions, exponential stretching and its consequence, asymptotic directionality, are the mechanisms controlling the evolution of chaotic reactive systems.
Propene bulk polymerization kinetics: Role of prepolymerization and hydrogen
Authors: Pater Jochem T. M. , Weickert Günter , van Swaaij Wim P. M.
Published: April 2004

An experimental setup for the polymerization of liquid propylene was used to carry out main polymerizations with and without a prepolymerization step. Two types of prepolymerization are introduced: at a constant temperature and at rapidly increasing reactor temperatures. With the present catalyst system, at high polymerization temperatures, a prepolymerization step will increase polymerization rate. Hydrogen was used to control the molecular weight of the product. The polymerization reactor was used in combination with a gas chromatograph to determine the vapor‐liquid equilibrium (VLE) of the propylene‐hydrogen system. Measurements are compared to predictions of different equations of state. The Peng‐Robinson equation of state is best able to describe the VLE for this binary system. A temperature‐dependent interaction parameter was derived from the fits to the measured data.
Nucleation and kinetics of mixed salts in scaling
Authors: Sheikholeslami Roya
Published: April 2004

During membrane and thermal desalination, the saline solution becomes supersaturated. Once the solution becomes unstable, nucleation and crystallization of the salts occur resulting in a buildup of an unwanted deposit layer on transfer surfaces and increasing the mass‐ or heat‐transfer resistance of the system and the energy input for producing water. Due to the process complexity, research focused primarily on single salt fouling and applying the nucleation and kinetics of a single system in developing physical models for precipitation (crystallization) fouling. This article demonstrates that single salt nucleation and crystallization kinetics applied so far to crystallization fouling cannot be extended to real situations when a combination of various salts and solutes are present. Other noncrystallizing species, if present, may affect the surface energy of a crystal and alter the Gibbs free energy for homogeneous nucleation. Other crystallizing species may also act as a nucleus initiating nonhomogeneous nucleation with a lower Gibbs free energy and promoting nucleation. The presence and catalytic and inhibitory effects of dissolved and solid species on metastability of the solution should be considered in kinetic analysis of crystallization fouling. Kinetic relationships developed take into account the kinetics of coprecipitating salts with and without a common ion and in the presence of other soluble species. They can be easily extended to multiprecipitating systems, but further advances in experimental techniques are necessary to integrate the kinetic analysis and experimental data.
Fuel options for solid oxide fuel cells: A thermodynamic analysis
Authors: Douvartzides S. L. , Coutelieris F. A. , Demin A. K. , Tsiakaras P. E.
Published: April 2004

The eligibility of natural gas (methane), methanol, ethanol, and gasoline as fuels for the generation of electrical power in solid oxide fuel cells (SOFCs) is discussed in terms of efficiency. Each raw fuel was assumed to be processed in a steam reformer to provide a hydrogen‐rich gas mixture to the SOFC feedstream. An SOFC system, consisting of an electrochemical section and a reformer, was analyzed thermodynamically assuming initial steam/fuel feed ratios at conditions where carbon deposition is thermodynamically impossible, at atmospheric total pressure, and in the temperature range of 800–1,200 K. Results were obtained in terms of both electromotive force (emf) output and efficiency. Methane seems to be the most appropriate fuel option, with an SOFC system efficiency close to 96%. Furthermore, ethanol and methanol were very promising alternative options (94% and 91%, respectively), while gasoline (83%) utilization requires special reforming conditions.
Multicomponent diffusion theory and its applications to polymer‐solvent systems
Authors: Price Peter E. , Romdhane Ilyess Hadj
Published: April 2004

Recent publications by Zielinski and Hanley and by Alsoy and Duda have proposed relationships between self‐ and mutual‐diffusion coefficients in multicomponent systems that can be derived from Bearman's friction‐based theory. Subsequently, Zielinski and Alsoy published an article calling into question the ability of these new theories to satisfy the Onsager reciprocal relations, and Nauman and Savoca also reported that the Zielinski and Hanley theory can, in some cases, fail to satisfy material balance constraints. Here, the existing friction‐based theories are reviewed and then previous approaches to relating self‐ and mutual‐diffusion coefficients are generalized. The result is an expression that both unifies previous results and is itself a previous result. This generalization prompts us to address the issues raised regarding the existing theories. Some example calculations are also presented, and some directions for future areas of research are suggested.
Model‐based measurement of diffusion using Raman spectroscopy
Authors: Bardow André , Marquardt Wolfgang , Göke Volker , Koss Hans‐Jürgen , Lucas Klaus
Published: April 2004

The measurement of diffusion coefficients in multicomponent liquid mixtures is still an open question. A new technique aimed at overcoming this deficiency is presented. It is based on Raman spectroscopy, which has been extended to obtain liquid concentration data of all components in a mixture simultaneously with high temporal and spatial resolution. The following data analysis uses model‐based methods. This provides a very flexible framework, allowing substantial reduction of experimental effort and time and the direct estimation of the diffusion coefficient as a function of concentration. Furthermore, the experimental procedure is improved using optimal experimental design techniques. This new methodology is validated for binary mixtures.
Effect of drag reducers on transport phenomena in turbulent pipe flow
Authors: Hamersma Peter J. , Fortuin Jan M. H.
Published: April 2004

Momentum transfer in turbulent pipe flows (d = 51 mm) with and without a drag reducer was studied. Age distributions of coherent fluid structures at the pipe wall were determined at different Reynolds numbers (104<Re<6 × 104) using laser‐Doppler velocimetry (LDV) and a suitable discriminating procedure. Mean ages, mean transfer thicknesses, friction factors, and time‐averaged axial velocity profiles derived from LDV signals were compared with those from axial pressure gradients using the random surface‐renewal model. The distribution of the ages was random, and drag reducers increased the mean age and the mean transfer thickness of the fluid elements at the wall, thus decreasing momentum and heat transfer at the fluid–wall interface. The mechanism of single‐surface renewals was derived from conditionally averaged, local axial fluid velocities obtained from LDV signals.
Nucleation regime map for liquid bound granules
Authors: Hapgood Karen P. , Litster James D. , Smith Rachel
Published: April 2004

Nucleation is the first step in granulation where the powder and liquid first contact. Two types of nucleation in wet granulation processes are proposed. Drop controlled nucleation, where one drop forms one nucleus, occurs when drops hitting the powder surface do not overlap (low spray flux Ψa) and the drop must wet quickly into the bed (short drop penetration time tp). If either criterion is not met, powder mixing characteristics will dominate (mechanical dispersion regime). Granulation experiments were performed with lactose powder, water, PEG200, and 7% HPC solution in a 6 L and a 25 L mixer granulator. Size distributions were measured as the drop penetration time and spray flux were varied. At short penetration times, decreasing Ψa caused the nuclei distribution to become narrower. When drop penetration time was high, the nuclei size distribution was broad independent of changes in dimensionless spray flux. Nucleation regime maps were plotted for each set of experiments in each mixer as a function of the dimensionless distribution width δ. The nucleation regime map demonstrates the interaction between drop penetration time and spray flux in nucleation. The narrowest distribution consistently occurred at low spray flux and low penetration time, proving the existence of the drop controlled regime. The nucleation regime map provides a rational basis for design and scale‐up of nucleation and wetting in wet granulation.
Model for temperature profiles in large diameter electrochromatography columns
Authors: Keim Craig , Ladisch Michael
Published: April 2004

Scale‐up of electrochromatographic separations has been problematic due to electrically induced heating. A two‐dimensional transient temperature model for electrochromatography was developed, which accounts for physical properties of the stationary and mobile phase, and the column wall. The model also accounts for both the temperature effect on the electrical conductivity and a nonuniform, radially variant current density. This model was compared to experimental data from two electrochromatography systems with different cylindrical‐column dimensions, packing materials, and operating conditions. In all cases, the model predicts the temperature to within 3°C of the actual temperature, both for column heatup and cooldown. Separation of a mixture of model proteins on the 3.81‐cm‐ID scale was used as the basis for scale‐up calculations. The model identifies equipment parameters that control heating characteristics and can be scaled up to process 75 mL of sample per run.
Fundamental thermal‐hydraulic pulp digester model with grade transition
Authors: Bhartiya Sharad , Dufour Pascal , Doyle Francis J.
Published: April 2004

A detailed fundamental model of a continuous pulp digester was developed for simulation and control. Most modeling efforts in the past had emphasized either reaction kinetics and energy transfer assuming a prescribed flow behavior or modeled digester hydraulics with simplified reaction kinetics. The well‐known extended Purdue model was augmented by incorporating axial momentum transport, of which an immediate consequence is the ability to simulate chip level and its impact on the Kappa number profile, thus enhancing the ability to model chip compaction. Interaction between digester hydraulics and kinetics is illustrated via simulation examples, including scenarios that show incipient plugging of the digester vessel. Higher‐order finite‐difference methods were also used to reduce numerical dissipation and dispersion. The ability to describe chip velocities enables rigorous tracking of a transition front within the digester during feedstock grade transitions. The model explains operational difficulties encountered during hardwood to softwood transition.
Optimal process design with model parameter uncertainty and process variability
Authors: Rooney William C. , Biegler Lorenz T.
Published: April 2004

Optimal design under unknown information is a key task in process systems engineering. This study considers formulations that incorporate two types of unknown input parameters, uncertain model parameters, and variable process parameters. In the former case, a process must be designed that is feasible over the entire domain of uncertain parameters, while in the latter case, control variables can be adjusted during process operation to compensate for variable process parameters. To address this problem we extend the two‐stage formulation for design under uncertainty and derive new formulations for the multiperiod and feasibility problems. Moreover, to simplify the feasibility problem in the two‐stage algorithm, we also introduce a KS constraint aggregation function and derive a single, smooth nonlinear program that approximates the feasibility problem. Three case studies are presented to demonstrate the proposed approach.
Fines deposition dynamics in gas–liquid trickle‐flow reactors
Authors: Iliuta Ion , Larachi Faïçal , Grandjean Bernard P. A.
Published: April 2004

Nonfilterable fines, such as incipient coke particles or fines naturally occurring in oil sands bitumen, are known to be responsible for the severe plugging during the flow of (fine) solid–liquid suspensions in cocurrent gas–liquid trickle‐bed hydrotreating reactors. Accumulating fines in the porous medium causes pressure buildup, and thus the dropoff in hydrogen partial pressure in the bed, overutilizing recycling compressors and shortening the reactor operating cycles. In this work, a 1‐D transient two‐fluid hydrodynamic model based on the macroscopic volume‐average form of the multiphase system transport equations is developed, analyzed, and validated experimentally. The model hypothesizes that plugging occurs via deep‐bed filtration mechanisms. It incorporates physical effects of porosity and effective specific surface‐area changes due to the capture of fines, inertial effects of phases, and coupling effects between the fines filter rate equation and interfacial momentum exchange force terms. It is tested in the trickle‐flow regime for conditions mimicking a hydrotreating trickle‐bed process with spherical and trilobe catalysts. To rationalize deep‐bed filtration phenomena in trickle‐flow reactors, parametric studies are carried out on the effects of liquid velocity and viscosity, gas density and velocity, and fines feed concentration, on the plugging dynamics.
Laminar mixing in different interdigital micromixers: II. Numerical simulations
Authors: Hardt S. , Schönfeld F.
Published: April 2004

Flow patterns and mixing properties of micromixing devices described in Part I are investigated by computational fluid dynamics (CFD) and semianalytical methods. The CFD results provide qualitative information on mixing, but, due to numerical artifacts, no quantitative information can be derived in most cases. The geometrical arrangement of liquid lamellae predicted by the CFD simulations explain transmitted‐light experiments reported in Part I. In contrast, the semianalytical method provides a simple model for the mixing devices presented and allows to assess their mixing properties. For two of the four micromixers, mixing times were in the range of milliseconds, thus lending themselves to fast, mass‐transfer‐limited reactions. Experimental data obtained for a specific micromixer support the semianalytical model. Both the model and experiments suggest that geometric focusing of a large number of liquid streams is a powerful micromixing principle.
Experimental analysis of hydrodynamics in a radially agitated tank
Authors: Escudié Renaud , Liné Alain
Published: April 2004

A PIV technique is used to analyze the local hydrodynamics generated by a Rushton turbine. Different types of motion coexist in the tank: the mean flow (or global circulation), the periodic fluctuations (or trailing vortices) induced by the blade rotation in the impeller region, and the turbulent fluctuations (that dissipate the kinetic energy). These three kinds of motion can be estimated after experiments as soon as a triple decomposition of the velocity is performed. The mean velocity, the periodically induced stress, and the Reynolds stress are analyzed in the agitated tank, close to the impeller. These data are used for two purposes: to identify and quantify the transfer of kinetic energy between mean flow, periodic flow, and turbulence; and to estimate the dissipation rate of turbulent kinetic energy (TKE) from the balance of TKE, in which each term will be derived from experiments. Characteristics of turbulence are also presented and discussed.
Mechanics of column beds: I. Acquisition of the relevant parameters
Authors: Yew Bee Gaik , Drumm Eric C. , Guiochon Georges
Published: April 2004

The efficiency of chromatographic columns is adversely affected by large‐scale radial variations of the packing density or void ratio of the material used to prepare the bed. This heterogeneity is due to wall friction effects that take place during the preparation of the column and to seepage effects operating during the packing process and the subsequent operation of the column. The dependence of the bed's void fraction on the stress applied during its consolidation was determined, as well as its permeability at various stages of the consolidation process and the coefficient of friction between typical packing materials and the stainless steel wall of chromatographic columns. These results are required to develop and use numerical models of the volumetric response to axial compression of the bed and models of the coupled mechanical‐seepage rheology of particulate materials.
Water‐density effects on phenol oxidation in supercritical water
Authors: Henrikson Jeffrey T. , Savage Phillip E.
Published: April 2004

Supercritical water oxidation (SCWO) experiments were performed in a tubular flow reactor at 420–465°C and 141–241 bar. Phenol was the organic reactant. Both pure water and a helium (1/3 by mol)–water mixture served as reaction media. Adding helium to the reaction medium permitted variation of the water concentration and system pressure independently. By decoupling the water concentration from the system pressure, it was shown that the rate of phenol disappearance during SCWO is influenced by the water concentration and not the system pressure. The experiments consistently revealed that adding helium, and thereby decreasing the water concentration at a fixed system pressure, increased the phenol conversion. In addition, experiments showed that lowering the water concentration using pure water as the solvent also increased the phenol conversion. The results suggest that dilution with helium (and perhaps other inert gases) may be a new way to control SCWO reaction rates.
Optimum photolysis in Taylor–Couette flow
Authors: Forney L. J. , Pierson J. A.
Published: April 2004

Photolysis was studied in Taylor–Couette flow, and a similarity law proposed for the formation of a product from a fast photochemical reaction in the presence of laminar Taylor vortices. The concentration of the product formed depended on both the dosage of photons and a reaction‐layer thickness, defined by the radiation penetration depth. In particular, for a given flow rate and radiation intensity, the UV photolysis of iodide produced a maximum concentration of the product triiodide, when the radiation penetration depth was equal to the velocity boundary‐layer thickness. The latter conclusion provides optimum operating conditions to maximize yield in terms of the Taylor number or frequency of rotation.
Monte Carlo simulation of adsorption using 2‐D models of heterogeneous solids
Authors: Cabral V. F. , Tavares F. W. , Castier M.
Published: April 2004

A new methodology proposed here correlates the adsorption of pure components and predicts the adsorption of binary and ternary mixtures in homogeneous and heterogeneous solids. This methodology uses the algorithm of molecular simulation in the grand canonical ensemble as an equation of state for the adsorbed phase. In all case studies presented, the simulations described the adsorption characteristics of systems. The results obtained for the adsorption of the binary mixtures of propane–CO2 and propane–H2S, which are strongly nonideal, were quite satisfactory, showing the potential of this technique for the description of real systems.
Diffusing with Stefan and Maxwell
Authors: Amundson Neal R. , Pan Tsorng‐Whay , Paulsen Vern I.
Published: April 2004

In most chemical engineering problems, diffusion is treated as an add‐on to forced advection, and the boundary conditions are the Danckwerts conditions in order to maintain conservation. If we treat problems in which there is no applied advection or chemical reaction for a steady‐state situation and with an ideal gas, it is an equi‐molar process. Because it is considered the natural movement of molecules moving down a concentration gradient, this requires the Stefan‐Maxwell equations. In the standard 1‐D two‐component boundary value problem, analogous to the Dirichlet problem, the solution is direct and is probably the only one there is. In a ternary system, the solution is already not direct and can only be obtained by numerical means, which is not severe. In a quaternary system, it does not appear feasible to obtain a simple procedure like that obtained for the ternary system. A different numerical scheme developed is robust with rapid convergence and works well with an arbitrary number of components, 60 having been used in one problem. As the number of components increases, the solution profiles tend to become linear and the dependence on particular diffusivities is less important. This manifests itself when using diffusivities from a random collection. The problem using a continuous distribution of components is solved, and computationally and theoretically the profiles are probably linear and with a single pairwise diffusion coefficient.
Multiscale resolution of fluidized‐bed pressure fluctuations
Authors: Zhao Gui‐Bing , Yang Yong‐Rong
Published: April 2004

Pressure fluctuation signals measured from four different axial locations in a bubbling bed 0.3 m in diameter and 3 m in height were analyzed using multiple approaches, including wavelet transform, Hurst analysis, multiscale resolution, and time‐delay embedding. After examining decomposition residuals using different compact support Daubechies wavelets, the Daubechies second‐order wavelet was chosen as an optimal wavelet for decomposing pressure signals. Hurst analysis of the decomposed signals shows that the measured pressure fluctuations can be resolved to three characteristic scales: bifractal mesoscale signals with two distinct Hurst exponents; monofractal micro‐ and macroscale signals with only one characteristic Hurst exponent. Energy profiles of the three scale components confirm that the measured pressure signals mainly reflect the mesoscale component. Time‐delay embedding analysis of three scale signals demonstrates that the microscale dynamics is more complex than the mesoscale dynamics, and the mesoscale dynamics is more complex than the macroscale dynamics. That this result cannot be found solely from Hurst analysis shows the importance of integrating multiple approaches for characterizing the complexity of fluidized systems.
One‐dimensional centrifugation model
Authors: Chan Steven H. , Kiang San , Brown Matthew A.
Published: April 2004

A 1‐D theoretical model based on Darcy's law and conservation of mass was used to describe transient filtration on a basket centrifuge for both compressible and incompressible cakes. This filtration model was validated assuming that a liquid layer lay above the surface of the cake. Both the resistance and porosity of the cake were assumed constant throughout the cake at a particular instant in time. A computational algorithm was developed to solve the system of nonlinear equations and to calculate pressure differentials across the cake, location of the slurry front in the basket, cake thickness, filtrate volume, cake resistance, cake porosity, and filtrate flow rate, all as functions of time. Both simulation and experimental results showed the validity of this computational model. A procedure was also developed to use small‐scale lab data, in conjunction with the model, to select the corresponding operating conditions for large‐scale equipment so that cake performance on both scales was dynamically similar. For the situation when the inlet feed had ceased and the liquid layer had fallen below the surface of the cake, the cake deliquoring model proposed by Wakeman and Vince was adopted.
Multiscale SPC using wavelets: Theoretical analysis and properties
Authors: Aradhye Hrishikesh B. , Bakshi Bhavik R. , Strauss Ramon A. , Davis James F.
Published: April 2004

Most practical process data contain contributions at multiple scales in time and frequency, but most existing SPC methods are best for detecting events at only one scale. For example, Shewhart charts are best for detecting large, localized changes, while EWMA and CUSUM charts are best for detecting small changes at coarse scales. A multiscale approach for SPC, adaptable to the scale of relevant signal features and developed based on wavelet analysis, detects abnormal events at multiple scales as relatively large wavelet coefficients. Univariate and multivariate multiscale SPC (MSSPC) for detecting abnormal operation are theoretically analyzed, and their properties are compared with existing SPC methods based on their average run lengths. SPC methods are best for detecting features over a narrow range of scales. Their performance can deteriorate rapidly if abnormal features lie outside this limited range. Since in most industrial processes, the nature of abnormal features is not known a priori, MSSPC performs better on average due to its adaptability to the scale of the features and for monitoring autocorrelated measurements since dyadic wavelets decorrelate most stochastic processes. MSSPC with dyadic discretization is appropriate for SPC of highly autocorrelated or nonstationary stochastic processes. If normal measurements are uncorrelated or contain only mild autocorrelation, it is better to use MSSPC with integer or uniformly discretized wavelets. Many existing methods such as MA, EWMA, CUSUM, Shewhart, batch means charts, and their multivariate extensions are special cases of MSSPC.
Development of an inherent safety index based on fuzzy logic
Authors: Gentile Michela , Rogers William J. , Mannan M. Sam
Published: April 2004

During the past several years, researchers in the U.S. and Europe have developed measurement techniques and analysis tools to estimate the inherent safety of a plant or a processing unit. These tools are based on traditional Boolean mathematical methodologies that are limited by the uncertain and subjective nature of the information analyzed. Preliminary results reported here are on the development of an inherent safety index based on fuzzy logic theory that is an extension of the Boolean theory. The basic ideas of fuzzy logic theory and its application are presented by the inherent safety evaluation of a storage tank. The inherent safety index developed by Heikkila is taken as a base and is compared with the results of the proposed prototype index. It shows the benefits of using fuzzy logic. Strengths and limitations of the proposed methodology are also presented.
Monitoring independent components for fault detection
Authors: Kano Manabu , Tanaka Shouhei , Hasebe Shinji , Hashimoto Iori , Ohno Hiromu
Published: April 2004

A chemical process has a large number of measured variables, but it is usually driven by fewer essential variables, which may or may not be measured. Extracting such essential variables and monitoring them will improve the process‐monitoring performance. Independent component analysis (ICA) is an emerging technique for finding several independent variables as linear combinations of measured variables. In this work, a new statistical process control method based on ICA is proposed. For investigating the feasibility of its method, its fault‐detection performance is evaluated and compared with that of the conventional multivariate statistical process control (cMSPC) method using principal‐component analysis by applying those methods to monitoring problems of a simple four‐variable system and a continuous‐stirred‐tank‐reactor process. The simulated results show the superiority of ICA‐based SPC over cMSPC.
Transport of organic components from immobile and bypassed oil in porous media
Authors: Huseby O. , Haugan A. , Sagen J. , Muller J. , Bennett B. , Larter S. , Kikkinides E. S. , Stubos A. K. , Yousefian F. , Thovert J.‐F. , Adler P. M.
Published: April 2004

An experimental study, as well as theoretical and numerical models, are used to validate a methodology to exploit conventional geochemical data with regard to the concentration profiles of organic components occurring naturally in hydrocarbon reservoir oils. The experiment was designed to study transport of organic compounds from bypassed oil during water injection using a homogeneous oil‐filled core sample, which was made heterogeneous by drilling a hole through its central axis and filling it with a highly permeable material. Under the present conditions, diffusion coefficients are the most important parameters controlling the transport, and the effect of partitioning could be accounted for by a simple normalization. The experimental results are well described by a simple 2‐D analytical model that assumes instantaneous removal of solutes from the oil–water interface. The experimental results are also well described by two numerical models, of which one is a full‐featured reservoir scale model, suitable for applications of the methodology to reservoir‐scale cases.
Finite‐element scheme for solution of the dynamic population balance equation
Authors: Rigopoulos Stelios , Jones Alan G.
Published: April 2004

A first‐order finite‐element scheme is proposed for solving the 1‐D dynamic population balance equation. The model includes the kinetic processes of nucleation, growth, aggregation, and breakage in a spatially uniform system with or without flow. The numerical scheme features improved stability by taking into account the convective nature of the growth term, and enhanced conservation of moments at nonuniform grids by identifying and mitigating the sources of conservation error in the evaluation of the aggregation source term. Exhaustive benchmarking is carried out against analytical solutions for size‐dependent growth and nucleation, size‐dependent aggregation with a variety of kernels, binary and multiple breakage with uniform and parabolic daughter particle distributions, and combinations of these, respectively. In all cases tested the method proves capable of producing accurate results at a reasonable CPU time, while being relatively simple and easy to implement.
Protein partitioning and transport in supported cationic acrylamide‐based hydrogels
Authors: Russell Shawn M. , Belcher Emily B. , Carta Giorgio
Published: April 2004

The partitioning and transport of myoglobin in cationic, acrylamide‐based hydrogels are studied by a microscopic visualization method. Homogeneous cationic gels are synthesized inside fused‐silica capillaries with a square section, which allow a direct determination of protein concentration profiles during transient adsorption and desorption. Diffuse, self‐similar profiles are observed and used to determine the equilibrium protein binding capacity and the protein diffusivity in the gel. Mass‐transfer rates are found to be essentially independent of the external solution concentration, but to vary dramatically with the gel polymer concentration. A Fickian diffusion model with a flux based on the adsorbed‐phase concentration gradient is consistent with the experimentally determined concentration profiles for both positive and negative protein concentration steps. The equilibrium and rate parameters determined for the capillary‐supported gels also compare favorably with those obtained from macroscopic measurements using composite ion‐exchange media comprising similar gels held within the pores of porous silica particles.
Stationary spatially complex solutions in cross‐flow reactors with two reactions
Authors: Sheintuch Moshe , Nekhamkina Olga
Published: April 2004

Formation of stationary spatially multiperiodic or even chaotic patterns is analyzed for a simple model of a cross‐flow reactor with two consecutive reactions and realistically high Le and Pe. Spatial patterns emerge much like dynamic temporal patterns in a mixed system of the same kinetics. The sequence of period doubling bifurcations is determined for the corresponding ODE system and is completely confirmed by direct numerical simulations of the full PDE model. The incorporation of a slow nondiffusing inhibitor led to chaotic spatiotemporal patterns.
Photolytic reactors: Similitude in Taylor–Couette and channel flows
Authors: Forney L. J. , Pierson J. A.
Published: April 2004

A similarity law is proposed for the yield from laminar photolytic reactors. In particular, the yield from a fast photolytic reaction is considered in the following three distinctly different reactor geometries: a square channel, an annular gap between concentric cylinders, and Taylor–Couette flow. The similarity law, based on the assumption of a self‐similar product concentration profile, demonstrates that the reaction rate can best be described with either zero (small λ) or first‐order (large λ) kinetics. Here, the distinction between both cases is determined by the radiation penetration depth, λ. Other geometric parameters affecting scale‐up are also discussed.
Leaching model for a cement mortar exposed to intermittent wetting and drying
Authors: Garrabrants Andrew C. , Sanchez Florence , Kosson David S.
Published: April 2004

Numerical simulations of inorganic species release from an intermittently wetted cement material without carbonation effects are presented. The intermittent mass‐transport (IMT) model combines previously developed approaches describing constituent diffusion coupled with solid‐phase dissolution/precipitation and two‐regime drying of a cement matrix stored at constant relative humidity. Model simulations were validated using experimental data showing the effects of intermittent wetting on release of major matrix species (Ca, Na, K, Cl, OH) and pH‐dependent trace metals (Cd, Pb). The IMT model successfully describes constituent release flux for intermittent wetting cases with storage in an inert atmosphere (100% N2) at three levels of relative humidity (23%, 48%, 98% RH). Leachate pH simulation accounted for the release of soluble alkaline salts in conjunction with the dissolution of calcium hydroxide. The release simulations for pH‐dependent species were subsequently improved over pH determination based solely on calcium hydroxide dissolution.
Liquid‐phase hydrogenation kinetics of isooctenes on Ni/Al2O3
Authors: Lylykangas M. S. , Rautanen P. A. , Krause A. O. I.
Published: April 2004

The kinetics of the hydrogenation of isooctenes [2,4,4‐trimethyl‐1‐pentene (TMP‐1), and 2,4,4‐trimethyl‐2‐pentene (TMP‐2)] to isooctane (2,2,4‐trimethylpentane) was studied on a commercial Ni/Al2O3 catalyst in a three‐phase reactor. Experiments showed that nickel is a highly active catalyst for the reactions. In addition to the hydrogenation reactions, slow isomerization occurred between TMP‐1 and TMP‐2. Kinetic equations were derived based on the Horiuti–Polanyi mechanism, involving a half‐hydrogenated surface intermediate and rate limitation in the first hydrogen addition. Isomerization was assumed to take place via the partly hydrogenated intermediate, since no isomerization products were observed in the absence of hydrogen. The dynamic reactor model consisted of material balances for the gas and liquid phases, as well as for the porous catalyst particles. The estimated activation energies for the hydrogenation of TMP‐1 and TMP‐2 were 34 and 49 kJ/mol, respectively. The model described the experimentally recorded data accurately.
Synthesis of methylacetate in a simulated moving‐bed reactor: Experiments and modeling
Authors: Lode Florian , Francesconi Gianmario , Mazzotti Marco , Morbidelli Massimo
Published: April 2004

Simulated moving‐bed reactors (SMBR) combine chemical reaction and adsorptive separation within one single unit. Toward the design of this rather complex unit, guidelines for the proper choice of the operating parameters, that is, the flow‐rate ratios and the Damköhler numbers within the different sections, have recently been devised based on a mathematical analysis of a linear‐model system. For nonlinear systems, though, the influence of the operating parameters on the unit performance is less well understood. In this work, the interplay between the operating conditions and unit performance is investigated through numerical simulations and experiments, focusing on the influence of the feed stream composition. As a model system, the synthesis of methylacetate from methanol and acetic acid, catalyzed by a sulfonated ion‐exchange resin, is considered. The experiments were carried out in an SMB reactor of miniplant scale. In addition, the reliability of the model predictions is evaluated by comparing the numerical simulation results with experimental data. It is shown through simulations that optimal performance is achieved where the feed to the SMBR is constituted of an equimolar mixture of the two reactants. Simulations and experimental results allow for an understanding of such a result.
Volume‐translated equations of state: Empirical approach and physical relevance
Authors: Yelash Leonid V. , Kraska Thomas
Published: April 2004

The volume translation technique is widely applied in chemical engineering modeling of phase behavior and thermodynamic properties. This empirical correction of the molar volume improves the correlation of liquid densities when cubic equations of state are employed. Based on a recently proposed method of analyzing equations of state, the influence of the volume translation on the properties obtained from an equation of state, such as virial coefficients, as well as on the phase behavior, critical properties, the isobaric thermal expansion coefficient, and the heat capacities of pure substances is investigated. The influence of different kinds of temperature‐dependent volume translations is investigated and compared to approaches that originate from theoretical considerations. The investigation shows that empirical as well as theoretical approaches for the development of equations of state can exhibit nonphysical behavior. However, the understanding of the reasons for such problems can help to develop reliable equations of state based on both kinds of method.
Coupled macromolecular transport and gel mechanics: Poroviscoelastic approach
Authors: Netti P. A. , Travascio F. , Jain R. K.
Published: April 2004

Designing drug delivery devices or drug delivery protocols poses problems because they are considered to operate in a mechanically static environment. It should consider, however, transport processes in these cases occur in a mechanically dynamic environment since mechanical stimuli may strongly influence transport within soft hydrated materials. A general framework combines fluid and macromolecular transport with the mechanics of hydrated polymer gels or tissue. As an example, the model equations have been created for a spherical geometry to describe the distribution of macromolecules within the gel resulting from a constant pressure or a constant flow infusion source. The model describes the overall average profiles of the interstitial fluid pressure, velocity and solid matrix dilatation, displacement, and stress, as well as macromolecular distribution. The basic theory provides new insight into understanding the transport of macromolecules within mechanically stimulated polymeric gels and tissues and, therefore, represents a valuable tool for designing and engineering novel drug delivery systems, as well as optimization of drug delivery protocols to be used in detection and treatments.
Entropic characterization of distributive mixing in polymer processing equipment
Authors: Wang Winston , Manas‐Zloczower Ica , Kaufman Miron
Published: April 2004

Mixing is an integral component of most polymer processing operations as material properties are highly influenced by the quality of mixing. The degree of distributive mixing (system homogeneity) is assessed by calculating the evolution of Renyi relative entropies for the minor component along a continuous processing equipment. The Renyi entropy involves a β parameter, which represents weighting given to the concentration of the minor component in small, localized regions. Different aspects of mixing can thus be analyzed, from the amount of void spaces to the concentration of the region where mixing is the worst in terms of the minor component. This method provides a unified, rigorous and flexible way of characterizing distributive mixing. Specifically, distributive mixing is analyzed in a numerical simulation of a twin‐flight single screw extruder by using particle tracking as a method of describing the mixing process dynamics. Renyi entropies are used to examine three different processing conditions in a twin‐flight, single screw extruder. By changing the throttle ratio, the optimal extruder length varied. The relationship between the optimal length and average residence time was good.
Nozzle‐quenching process for controlled flame synthesis of titania nanoparticles
Authors: Wegner Karsten , Pratsinis Sotiris E.
Published: April 2004

A process for precisely controlled synthesis of nanoparticles with a broad range of sizes, morphologies, and phase compositions is presented. This is achieved by a rapid quenching of the entire flame aerosol in a critical‐flow nozzle placed above and into the flame. This process is evaluated for synthesis of titania nanoparticles by oxidation of titanium‐tetra‐isopropoxide (TTIP) in a methane/oxygen coflow diffusion‐flame reactor. Precise control of phase composition from 97 to 5 wt. % anatase (and the balance rutile) and average primary particle diameter from 5 to 60 nm is possible by positioning the quenching nozzle at the desired heights above the burner and controlling gas and precursor flow rates. The nozzle quenching also reduces the degree of agglomeration of the product particles. An operation diagram shows that the primary particle diameter and the phase composition can be independently controlled, making anatase or rutile nanoparticles with high or low specific surface area.