bluuebunny/crossref_metadata_2025 huggingface dataset:
Mean concentrations and concentration fluctuations in a stirred‐tank reactor
Authors: Verschuren Iris L. M. , Wijers Johan G. , Keurentjes Jos T. F.
Published: April 2004

Mixing has a major influence on the product ratio of fast competitive reactions, as the product ratio of these reactions is determined by local concentrations. To integrate models from literature for the description of the mean and fluctuating concentration of a continuous reactant flow fed to a stirred‐tank reactor, the models are validated against detailed experimental data for a stirred‐tank reactor equipped with a Rushton turbine impeller. Application of these models requires information on local hydrodynamic parameters, which are determined by laser Doppler velocimetry. Model calculations are validated by determining the mean concentration and concentration variance of an inert tracer fed with planar laser‐induced fluorescence to the studied stirred‐tank reactor. To calculate the mean concentration, a combination of a theoretical model, measured mean concentrations and LDV measurements is used to determine the turbulent diffusion coefficient. The turbulent mixer model is used to calculate the concentration variance. Compared to the literature, this model requires adjustment of the constant in the production of the concentration variance to fit the measured concentration variances correctly. For the mean concentration and its variance, measurements agreed reasonably with simulations.
Fluidized‐bed pyrolysis of polyolefins wastes: Predictive defluidization model
Authors: Mastellone Maria Laura , Arena Umberto
Published: April 2004

The risk of defluidization during the low‐temperature pyrolytic processes of polyolefins is one of the major constraints to a wider utilization of fluidized beds for the pyrolysis of plastic wastes. A predictive model can estimate the occurrence of defluidization under different operating conditions, taking into account the role of physical properties of the polymers and hydrodynamic variables of the reactor, together with that of the main operating parameters, like the process temperature and the plastic waste feed rate. The results agree well with the experimental data obtained by feeding recycled polypropylene and polyethylene in a laboratory‐scale bubbling fluidized‐bed pyrolyzer. An operating map allows to distinguish the region of reactor stable operation from that of defluidization.
Effect of compressibility on performance of hydraulic wash columns
Authors: van Oord‐Knol L. , Bruinsma O. S. L. , Jansens P. J.
Published: April 2004

In a hydraulic wash column the solid crystals are separated from the mother liquor by filtration in the top section of the column. Remaining impurities are removed via countercurrent washing of the crystals in the bottom section. Since compressibility limits the capacity of a wash column, this phenomenon needs to be quantified and modeled rigorously. The compressibility of the bed was determined from the porosity profile and the liquid‐pressure profile inside the wash column. The porosity of the bed decreases from 0.65 to 0.3 during transport of the bed. This is associated with a decrease in local bed permeability by a factor of 10. The compressibility of the bed, therefore, partly explains the large ratio between the average permeability above and below the wash front. Compressibility coefficients make it possible to relate the compressive stress to the porosity and permeability in the top section of the bed. These coefficients are, therefore, incorporated in a model to successfully predict the capacity of a wash column with a compressible bed.
Thermal boundary layers in sweeping gas membrane distillation processes
Authors: Khayet M. , Godino M. P. , Mengual J. I.
Published: April 2004

The sweeping gas membrane distillation process was analyzed in a plate and frame membrane module, and a method clarifying the contribution of each boundary layer separately was developed. The temperature polarization coefficients of the fluid phases as well as the overall one have been defined, and their effects on the permeate flux were studied. Also studied is the influence of experimentally relevant parameters, such as the inlet temperatures or circulation velocities of the fluids. Results from two porous and hydrophobic membranes in different experimental conditions were interpreted on the basis of a combined Knudsen/molecular diffusion flow model. The theory agreed well with the experiment.
Ministructured catalyst bed for gas‐liquid‐solid multiphase catalytic reaction
Authors: Liu Wei
Published: April 2004

A ministructured reactor concept improves the performance of multiphase gas‐liquid‐solid reactions over conventional packed‐bed and trickle‐bed reactors. In the mini‐structured reactor, the catalyst bed is structured at the particle level by dividing the bed into a number of identical, parallel flow channels with the channel walls being catalyzed. The hydraulic diameter of each channel opening is from 0.2 to 2 mm, comparable to the conventional particle size. The mini‐structured reactor can be assembled using monolith catalyst modules that have been a proven material technology for gas‐phase reactions. Two types of gas‐liquid‐solid reactions, olefin hydrogenation and toluene saturation, are used to test the fundamental feasibility of the mini‐structured reactor under steady‐state reaction conditions in a cocurrent down flow mode. The new reactor shows an order of magnitude increase in the activity compared to a trickle‐bed operation for reaction conditions of practical interest. This degree of activity improvement is larger than estimates based on conventional analyses of catalyst geometric surface area and/or characteristic pore diffusion lengths. Individual catalyst particles are significantly underutilized in a trickle‐bed reactor. More ideal contacting of gas, liquid and catalyst, as well as the enhancement of overall mass‐transfer processes in the mini‐structured reactor, makes it possible to dramatically improve the gas/liquid multiphase reaction performance over solid catalysts.
Role of water in the kinetic modeling of catalyst deactivation in the MTG process
Authors: Gayubo Ana G. , Aguayo Andrés T. , Morán Angel L. , Olazar Martín , Bilbao Javier
Published: April 2004

A kinetic model is proposed for deactivation by coke of a HZSM‐5 zeolite catalyst used in the transformation of methanol into hydrocarbons in the 573–723 K range. The attenuating effect on coke deposition due to the presence of water in the reaction medium was quantified by a parameter that increases with reaction temperature. The slower deactivaton of the methanol dehydration step compared to the formation steps of olefin and gasoline lumps is taken into account. In view of the results, water content in the reaction medium is an efficient variable for controlling the results of activity–selectivity–deactivation. This model is not applicable for a temperature of 723 K when water content in the reaction medium is high, due to the fact that under these conditions the zeolite is subjected to irreversible deactivation.
CFD modeling and simulation of clogging in packed beds with nonaqueous media
Authors: Ortiz‐Arroyo Arturo , Larachi Faïçal , Grandjean Bernard P. A. , Roy Shantanu
Published: April 2004

When liquids containing low concentrations of fine solid impurities are treated in packed‐bed reactors, clogging develops and starts hampering the flow severely. This phenomenon, called deep‐bed filtration, constitutes serious concern over hydrotreating or hydrocracking of bituminous sands in packed‐bed reactors, in which such nonfilterable fines as native clay or incipient coke cause reactor dysfunction by clogging. A detailed k‐fluid Eulerian 2‐D transient computational‐fluid dynamic (CFD) model was formulated to describe the space‐time evolution of clogging patterns developing in deep‐bed filtration of the liquids. A local formulation of the macroscopic logarithmic filtration law is proposed, as well as a geometrical model for the effective specific surface area of momentum exchange. Both mono‐ and multiple‐layer deposition mechanisms were accounted for by including appropriate filter coefficient formulations. Transient, 2‐D axisymmetrical simulations were benchmarked using experimental results and observations of Narayan et al. (1997) of the carbon‐black contaminated kerosene flow through packed beds. Comparing the simulations and experiments showed that CFD is useful for the quantitative description of packed‐bed clogging.
Multicomponent diffusion of hydrogen‐bonding solutes in a polymer
Authors: Elabd Yossef A. , Barbari Timothy A.
Published: April 2004

The diffusion of methyl ethyl ketone (MEK)/1‐butanol (BOH) mixtures in polyisobutylene (PIB) was studied using time‐resolved FTIR‐ATR spectroscopy at varying mixture compositions. Diffusion coefficients and hydrogen‐bonding interactions were quantitatively measured using this technique. In the MEK/BOH/PIB system, three species were identified from the infrared spectra, free MEK, an MEK‐BOH complex, and a self‐associated BOH cluster. The diffusion coefficient for the MEK‐BOH complex was less than that for free MEK, but larger than the values determined for the BOH cluster. These results were compared to a numerical solution of a multicomponent transport model that explicitly accounts for solute–solute interactions to determine individual diffusion coefficients for all the species identified. In addition, the MEK/BOH/PIB results were compared to those for an MEK/acetonitrile (ACN) mixture, a nonhydrogen‐bonding system, in PIB, to elucidate the differences between interacting and noninteracting systems.
Dynamic behavior of hydraulic wash columns
Authors: van Oord‐Knol L. , Bruinsma O. S. L. , Jansens P. J.
Published: April 2004

Hydraulic wash columns are well suited for solid–liquid separation and ultrapurification in melt‐suspension crystallization. For steady operation of a hydraulic wash column and proper purification, the length of the crystal bed and position of the wash front should stay within safe limits, which requires a well‐defined control system. To understand the interactions between slurry properties, column dimensions, and process conditions, a dynamic model was developed. The model was validated with help of experiments in a pilot‐plant wash column, fed from a continuous crystallizer. P‐Xylene was used as the model compound. It satisfactorily predicted the trends in the length of the crystal bed and the wash front height, and the interactions in the wash column are largely understood. With the help of the model a feedback control system with two control loops was designed and simulated. For a wide range of crystal fractions (0.05 to 0.3 v/v) in the feed stream, both the length of the bed and the wash front were kept at the desired setpoint values, indicating the flexibility of a controlled wash column.
Backstepping design of composition cascade control for distillation columns
Authors: Alvarez‐Ramirez Jose , Alvarez Jesus , Monroy‐Loperena Rosendo
Published: April 2004

Although the cascade control of distillation columns had been extensively studied, some aspects still remain unexplored or unresolved. For instance, a unifying approach is needed to systematize the existing ad hoc controller constructions, to rigorously explain their remarkably robust property in using rather simple input‐output linear models, and to explore the possibility of improving their construction and functioning. Some of these problems are addressed by the recently developed backstepping‐passivation approach in constructive nonlinear control, yielding a systematic controller construction coupled to a simple tuning scheme that can be executed with standard tuning rules, a closed‐loop stability criterion, an explanation of the closed‐loop dynamics behavior, and controllers that perform better than existing ones. The applicability of the theoretical results and their controller construction‐tuning schemes are illustrated by considering the single‐ and dual‐point control cases of a binary acetone‐ethanol distillation column.
Absorption with instantaneous reaction in a droplet with sparingly soluble fines
Authors: Scala Fabrizio , D'Ascenzo Michele
Published: April 2004

The problem of gas absorption followed by an instantaneous irreversible chemical reaction in a rigid droplet containing sparingly soluble fine reactant particles is discussed. A model was developed for the general case when the reactant particle dissolution near the gas–liquid interface is nonnegligible. Solids dissolution is treated taking into account the average spacing between the suspended particles. A simple analytical expression for the rate of gas absorption is derived under the assumption of slow particle dissolution, and conditions are given for applicability of the model. A numerical example, relevant for the spray‐dry desulfurization process, for the system SO2‐Ca(OH)2 in water is discussed.
Hybrid framework for hazard identification and assessment in batch processes
Authors: Viswanathan S. , Shah N. , Venkatasubramanian V.
Published: April 2004

A framework presented addresses the three main parts of a process hazards analysis study: hazard identification, hazard evaluation, and hazard mitigation. The framework utilizes a hybrid methodology that effectively combines a qualitative digraph model‐based technique for performing hazard identification and a quantitative optimization‐based technique for performing hazard evaluation and hazard mitigation. The combined framework circumvents the local minima problems faced by mathematical optimization algorithms, as well as the qualitative ambiguities that plague digraph models. The systems used to implement the framework are BatchHAZOPExpert, gPROMS, and gOPT. BatchHAZOPExpert is used for hazard identification, while gPROMS and gOPT are used for hazard evaluation and hazard mitigation. The application of the integrated framework is illustrated by implementing it on two industrial case studies.
Role of particle geometry and surface contacts in solid‐phase reactions
Authors: Kostogorova J. , Viljoen H. J. , Shteinberg A.
Published: April 2004

Given the surface areas of three different species A, B, and C, what is the most likely contact area between A and B? This problem finds many applications, but it is of specific importance in solid‐phase reactions. Reactions in powder mixtures depend strongly on contact area between reactants, even when one species may melt. The surface of particles is meshed with small “tiles,” and a combinatorial problem is formulated to map all tiles onto each other. The number of different contacts of n constituents is \documentclass{article}\pagestyle{empty}\begin{document}$\sum\limits_{i = 1}^n i$\end{document}; if pores are present, they are considered a constituent. The combinatorial problem for n > 3 is computationally overwhelming, but for two or three species, the desirable contacts can be calculated. The model was developed for contact between three different species (two species are included as a special case). This could constitute three different powders at 100% MTD, or a mixture of two powders that includes pores where the latter phase acts as a third species. The combinatorial approach is used to find the discrete probability‐distribution function (PDF), viz., p(z, A, B, C), where z is the number of desirable contacts (for example, A∖︁B), given the surface areas (A, B, C). The first moment of the PDF gives the expectancy value Ψ (A∖︁B, A, B, C) for contact between species A and B. The theory was demonstrated by two examples. A simple contact problem is solved for two powders that also contain pores. The second example compares kinetic rates for different shapes and sizes of particles.
Linear control of nonlinear systems: Interplay between nonlinearity and feedback
Authors: Eker S. Alper , Nikolaou Michael
Published: April 2004

A rigorous and general theory, as well as an associated efficient computational methodology, was developed that addresses the question of when and what linear control is adequate for a nonlinear process. A number of computer simulation examples illustrate the power of the proposed approach. Several potential future developments are outlined.
LF‐LPV input/output data‐based predictive controller design for nonlinear systems
Authors: Yoo Kee‐Youn , Rhee Hyun‐Ku
Published: April 2004

Most control engineers concentrate on finding a controller given the plant model or identifying a model from the data. There is no doubt that model‐based control and system identification are closely related, simply because one depends strongly on the other. In this work a subspace identification algorithm for LF‐LPV (linear‐fractional linear parameter‐varying) models is reformulated from a control point of view. This algorithm is referred to as an input/output data‐based predictive control, in which an explicit model of the system to be controlled is not calculated at any point in the algorithm. It allows for the construction of a nonlinear model predictive controller for an unknown nonlinear system directly from a set of its open‐loop measurements. As an example of the input/output data‐based predictive control, the styrene solution polymerization in a continuous reactor system is considered to prove the superior performance of LF‐LPV input/output data‐based predictive controller for polymer quality control. This approach gives a new angle for attacking the problem of identifying and controlling nonlinear systems.
Design of homogeneous biphasic catalytic processes
Authors: Singh Divya J. , Samant Ketan D. , Ng Ka M.
Published: April 2004

A systematic procedure for the design of homogeneous catalytic processes with special emphasis on biphasic reactions was developed. First, the reaction kinetics were determined, and then a generic stirred cell model for biphasic homogeneous catalytic reactions was used to elucidate the interplay of reaction and mass transfer and the impact of phase equilibria. The optimal range of reactor conditions, as captured by the Damköhler numbers, was then related to reactor attributes. Process alternatives for catalyst recovery and recycle were considered.
Pattern matching in historical data
Authors: Johannesmeyer Michael C. , Singhal Ashish , Seborg Dale E.
Published: April 2004

For many engineering and business problems, it would be very useful to have a general strategy for pattern matching in large databases. For example, the analysis of an abnormal plant condition would benefit if previous occurrences of the abnormal condition could be located in the historical data. A new pattern‐matching strategy is proposed for multivariate time series based on statistical techniques, especially principal‐component analysis (PCA). The new approach is both data‐driven and unsupervised because neither training data nor a process model is required. Given an arbitrary set of multivariate data, the new approach can be used to locate similar patterns in a large historical database. The proposed pattern‐matching strategy is based on two similarity factors: the standard PCA similarity factor and a new similarity factor that characterizes the pattern of alarm violations. An extensive simulation study for a chemical reactor demonstrates that this strategy is more effective than existing PCA methods and can successfully distinguish between 28 different operating conditions.
Nucleation, growth, and agglomeration in barium sulfate turbulent precipitation
Authors: Marchisio Daniele L. , Barresi Antonello A. , Garbero Mirko
Published: April 2004

In this work barium sulfate precipitation is studied in a tubular reactor in a wide range of operating conditions. The effect of reactant concentrations and barium or sulfate excess on crystal‐size distribution and morphology is investigated. Experimental results show that ion excess has a strong influence and that at high concentration aggregation takes place. Computational fluid dynamics is coupled with the finite‐mode probability density function approach for taking into account both macro‐ and micromixing, whereas the population balance is treated by using the standard moment method. Comparison with experimental data suggests that when micromixing and agglomeration are properly taken into account, the agreement is improved. However, this statement is partially affected by the lack of knowledge in barium sulfate kinetics.
Improvement of hydrogen storage by adsorption using 2‐D modeling of heat effects
Authors: Delahaye A. , Aoufi A. , Gicquel A. , Pentchev I.
Published: April 2004

The use of hydrogen storage based on an adsorption process for cars operating on a fuel cell will be seriously considered when the amount of stored gas is high enough, the tank safe and economically viable, and the filling process fast enough. To maximize the storage capacity for rapid filling, it is necessary to minimize heating of the adsorbent bed and to imagine means to extract energy toward the outside. Numerical simulation of such a hydrogen storage tank, based on a 2‐D model where mass and thermal energy balances are coupled, is presented. Results from simulations agreed well with experiments, when various modified parameters were calculated. Solutions for improving adsorbent beds for hydrogen storage tanks are then proposed.
Mechanisms of mixing and creation of structure in laminar stirred tanks
Authors: Alvarez M. M. , Zalc J. M. , Shinbrot T. , Arratia P. E. , Muzzio F. J.
Published: April 2004

Although stirred tanks have been the most commonly used fluid mixing devices since the beginnings of the industrial era, little is known about how and why they mix, and, therefore, about how to improve their performance, particularly when operated in the laminar regime. In laminar conditions, chaos is the only route to achieve efficient mixing. From the body of research in 2‐D chaotic flows, very minute perturbations in the velocity field can lead to widespread chaos and substantial enhancement of mixing performance, but these observations have not been systematically applied to industrially relevant 3‐D flows. Using planar laser induced fluorescence and direct numerical simulations, the mechanisms of creation and evolution of mixing structures were studied in Newtonian and non‐Newtonian flows. The observed dynamical behaviors are related to geometric features of the system. In Newtonian flow systems, the passing of the impeller blades triggers the onset of chaos by introducing small perturbations to the underlying regular 2‐D flow that is observed when impellers are substituted with discs. In non‐Newtonian viscoelastic systems, nonlinearity in the stress field introduced by the fluid rheology drives the system to spontaneous chaos even when concentric discs are used to stir the fluid.
Tracer mixing dynamics during aggregation and fragmentation
Authors: McCoy Benjamin J. , Madras Giridhar
Published: April 2004

For many chemical systems consisting of particles, liquid droplets, or gas bubbles undergoing concurrent (reversible) aggregation and fragmentation in a stirred immiscible fluid, evolution of the particle distribution and mixing of chemical species are important fundamental issues. A dynamic model for dispersive mixing systems presented here illustrates the time dependence of the particle‐size distribution (PSD) and the tracer mass distribution (TMD). For rapid tracer diffusion, the population balance equations (PBEs) for the PSD and TMD are identical (only the initial conditions differ) for binary aggregation and fragmentation rate coefficients that do not depend on particle size or tracer mass. The integrodifferential PBE is solved by a numerical method, and results agree with the moment solution. Measures for the degree of tracer mixing are related to the variance or polydispersity index and to the mixing entropy. The kinetics are reversible, allowing the PSD, TMD, and mixing entropy to approach stationary states. Results are partially compared with a similarity solution for the PSD.
Monitoring of batch processes using spectroscopy
Authors: Gurden Stephen P. , Westerhuis Johan A. , Smilde Age K.
Published: April 2004

There is an increasing need for new techniques for the understanding, monitoring and the control of batch processes. Spectroscopy is now becoming established as a means of obtaining real‐time, high‐quality chemical information at frequent time intervals and across a wide range of industrial applications. In this article, the role of spectroscopy for batch process monitoring is discussed in terms of both current and potential advances. The emphasis is on how to handle the measured data to extract maximum information for improved process performance and efficiency. In particular, the use of spectroscopy for statistical process monitoring is detailed and considered as complementary to the use of engineering process data. A case study of the ultraviolet‐visible monitoring of a first‐order biochemical conversion reaction is described, as well as the advantages of spectroscopy for process fault detection and diagnosis. Future prospects for the use of on‐line spectroscopy are also discussed.
Foaming mechanisms in surfactant free particle suspensions
Authors: Bindal S. K. , Sethumadhavan G. , Nikolov A. D. , Wasan D. T.
Published: April 2004

Physical mechanisms for foam formation in the presence of colloidal particles are poorly understood. This study is aimed at enhancing the basic understanding of the mechanisms that produce foaming in a three‐phase system, containing solid, liquid, and gas, and identifying key parameters that aggravate foaming in the absence of surface‐active agents. The foaming ability of a system (“foaminess”) was investigated by aerating a suspension of nanosized hydrophilic silica particles. We observed that the foaminess was directly proportional to particle concentration and inversely proportional to particle size. However, even a small degree of bidispersity in particle size drastically reduced foaminess. To explain these observations, the drainage of a single foam lamella containing nanosized particles was studied using a novel capillary force balance apparatus, which incorporates the microinterferometric technique. The single foam lamella thins in a stepwise manner, as the colloidal particles self‐organize into a layered structure between the gas bubbles. This layer provides a barrier against coalescence of bubbles, thereby stabilizing the foam lamella. The number of stepwise film thickness transitions increased with particle concentration and decreased with the increase in the particle size, consistent with the results from foaming experiments. Particle bimodal distribution greatly decreases the foam‐lamella stability, consistent with the theoretical prediction.
Interpretation of ethane hydrate equilibrium data for porous media involving hydrate‐ice equilibria
Authors: Zhang Wu , Wilder Joseph W. , Smith Duane H.
Published: April 2004

Equilibrium pressures for the dissociation of ethane hydrate confined in silica gel pores of nominal radii 7.5, 5.0 or 3.0 nm were measured over a range of temperatures. Each of these porous media contained a broad distribution of pore radii. At higher temperatures, the pressures were larger than previously reported for bulk hydrates, and generally increased with a decrease in nominal pore radius for those data where the equilibria involved liquid water. However, at the lowest temperatures (where the equilibria involved ice), the pressures were identical (within expected experimental uncertainties) for all three silica gels and the same as reported by various authors for the bulk hydrate. The lack of any dependence of the equilibrium pressure on pore size at the lowest temperatures (where the equilibrium involves ethane hydrate, ethane gas, and ice) indicates that, below the quadruple point temperature of the smallest pore in which hydrate formed, there is no detectable effects on the equilibrium pressure due to the restricted geometries of the porous media. It suggests that the interface relevant to the formation of hydrate in silica gel pores is that between the hydrate and aqueous phase (Henry et al., 1999) and that within experimental error the surface energy between the hydrate and aqueous phase can be approximated by that between ice and the appropriate aqueous phase.
Colloid‐associated contaminant transport in porous media: 2. Mathematical modeling
Authors: Sen Tushar Kanti , Nalwaya Nitesh , Khilar Kartic C.
Published: April 2004

A comprehensive mathematical model developed describes the colloid‐associated contaminant transport in groundwater flows based on equilibrium adsorption of contaminants, hydrodynamic release, migration, and capture of colloidal fines in groundwater flows. This model is more realistic than the existing models in that the physics of migration of colloidal fines is adequately incorporated. It also accounts for the entrapment and plugging of fine particles and, therefore, predicts both facilitation and retardation of the transport of contaminants depending on the flow and other conditions. The conditions leading to inhibition/retardation of contaminant transport due to plugging are: high release coefficient, low initial porosity, high inlet fines concentration, and high initial permeability. The mobilization and migration of colloidal fines, kaolin and their role on plugging, as well as on Ni2+ contaminant transport through kaolin‐sand packed beds, were studied, and the model was tested with published experimental measurements, as well as with our laboratory column results. The plugging‐based retardation of contaminant transport can be used to develop a new containment technique.
Modeling intra‐ and extra‐particle processes of wood fast pyrolysis
Authors: Di Blasi Colomba
Published: April 2004

A detailed single‐particle model, including a description of transport phenomena and a global reaction mechanism, is coupled with a plug‐flow assumption for extraparticle processes of tar cracking, in order to predict the fast pyrolysis of wood in fluid‐bed reactors for liquid‐fuel production. Good agreement is obtained between predictions and measurements of product yields (liquids, char, and gases) as functions of temperature. Particle dynamics are very affected by the convective transport of volatile products. The average heating rates are on the order of 450–455 K/s, whereas reaction temperatures vary between 770 and 640 K (particle sizes of 0.1–6 mm and a reactor temperature of 800 K). The effects of several factors, such as size, shape, and shrinkage of wood particles, and external heat‐transfer conditions are also examined.
Hydrodynamic transport properties of concentrated suspensions
Authors: Chang Chingyi , Powell Robert L.
Published: April 2004

A method of estimating the dependency of the relative viscosities and average sedimentation velocities of particle dispersions on the particle packing behavior, which is related to the shape (sphericity or aspect ratio) of the particle and the ruggedness of the particle surface is proposed. For each ensemble of microstructures, as determined by the flow system in which measurements are made (such as steady shear flow, high‐frequency dynamic shear flow, or sedimentation in a static fluid), a single master curve is obtained. These curves allow direct estimates of the relative viscosities or sedimentation velocities to be made if the maximum packing fraction is known.
Fluid mechanics of laminated sheet manufacture
Authors: Eames I. , Wright N. , Gilbertson M. A.
Published: April 2004

The fluid mechanics of laminated sheet manufacture was studied by pouring a liquid filler from a point source onto an inclined moving sheet and then passing it under a narrow gap below a pinch bar that spreads the liquid under the influence of gravity. The liquid filler must pass under the pinch bar before it starts to solidify. The spreading of a viscous liquid as it undergoes this process was studied theoretically and experimentally. Steady similarity solutions were obtained based on a vertically averaged formulation of the mass and momentum equations for viscous flows. Experimental measurements of the spread of the fluid and the time taken for fluid to travel from the source to the pinch bar agreed well with theoretical predictions. The new models are applied to the operation of the manufacturing process and the implications are discussed.
Analysis on nanoparticle growth by coagulation in silane plasma reactor
Authors: Kim Dong‐Joo , Kim Kyo‐Seon
Published: April 2004

The particle growth in a silane plasma reactor by coagulation between particles was analyzed by using a discrete‐sectional model, considering effects of a particle charge distribution based on the Gaussian distribution function. At the start of the plasma discharge, there is the high concentration of small‐size particles, and later large‐size particles appear and grow by coagulation between small‐size particles. Some fractions of small‐size particles are in a neutral state or even charged positively. The positively charged small‐size particles coagulate faster and more selectively with large‐size particles, which are charged more negatively than with medium‐size particles. Also, the particle‐size distribution in a plasma reactor becomes bimodal and large‐size particles become quite monodisperse. As the mass generation rate of monomers increase or as the monomer diameter decreases, large‐size particles grow more quickly and the particle‐size distribution becomes bimodal earlier. The plasma reactor can be a good candidate to produce monodisperse, nanosized particles.
Intrapellet diffusivities from TAP pulse responses via moment‐based analysis
Authors: Colaris A. H. J. , Hoebink J. H. B. J. , de Croon M. H. J. M. , Schouten J. C.
Published: April 2004

Moment‐based analysis was applied to determine successfully inter‐ and intrapellet diffusion coefficients from experimental TAP outlet pulse responses over a one‐zone bed with porous particles. Different temperatures, gas molar masses, and pellet sieve fractions were used to validate the Knudsen diffusion relation for inert gases inside porous catalyst pellets. The zeroth to third moments of the outlet pulse response were analytically derived for rectangular, cylindrical and spherical pellets. The third moment provides a significantly more accurate access to the intrapellet diffusivity than the second moment does. The experimental window of the TAP reactor is explored for porous silica as catalyst support bed filling. The relevance of concentration gradients inside the pellet is illustrated with model simulations.
Effects of diffusion‐controlled reactions on atom‐transfer radical polymerization
Authors: Delgadillo‐Velázquez Omar , Vivaldo‐Lima Eduardo , Quintero‐Ortega Iraís A. , Zhu Shiping
Published: April 2004

A kinetic model incorporating effects of diffusion‐controlled reactions on atom‐transfer radical polymerization (ATRP) was developed. The reactions considered to be diffusion‐controlled are monomer propagation, bimolecular radical termination, chain transfer between propagating radical and catalyst, and transfer to small molecules. Model predictions indicate that a diffusion‐controlled propagation reduces the “living” behavior of the system, whereas a diffusion‐controlled termination enhances its livingness. When diffusion‐controlled transfer between chains and catalyst is considered the same in the forward and backward directions, the livingness of the system is enhanced, but if one of them is kept unchanged while the other is increased, the livingness of the system is reduced. When diffusion‐controlled termination is important, our simulations show that the overall effect of diffusion‐controlled phenomena in ATRP is to enhance the livingness of the system. Experimental data from the literature for styrene, methyl methacrylate, and methyl acrylate ATRP homopolymerizations validate the kinetic model.
Continuous atom transfer radical block copolymerization of methacrylates
Authors: Shen Youqing , Zhu Shiping
Published: April 2004

A column reactor packed with silica‐gel‐supported copper bromide (CuBr)–hexamethyltriethylenetetramine (HMTETA) was developed for the continuous‐transition–metal‐mediated living/controlled radical polymerization [atom transfer radical polymerization (ATRP)] for homo‐ and block copolymerization of methyl methacrylate (MMA). The reactor showed good stability (>100 h) in both catalyst activity and molecular weight of the poly(methyl methacrylate) (PMMA) produced. The MMA conversion decreased as the flow rate increased. The polymerization in the reactor was in first order with respect to monomer concentration. The molecular weight of the resulting PMMA increased linearly with MMA conversion, exhibiting the characteristic feature of living polymerization. Thus, simply changing the flow rate of MMA, which in turn determined the MMA conversion, readily controlled the molecular weight of PMMA. The block copolymerization of MMA with n‐butyl methacrylate (nBMA) was carried out using two reactors connected in series. The block copolymers were slightly contaminated by PMMA prepolymer from the first column. The molecular weight of the nBMA block could be changed by the flow rate of the nBMA in the second reactor. This packed column reactor technology is feasible for continuously synthesizing polymers or block copolymers with controlled molecular weights that are readily adjustable by changing the monomer flow rates.
A theoretical model for quiescent coarsening in immiscible polymer blends
Authors: Hu Guo‐Hua , Li Huxi , Feng Lian‐Fang
Published: April 2004

The coalescence rate in immiscible polymer blends predicted by all existing models is two or three orders of magnitude lower than that observed experimentally. The model developed in this study considers for the first time that coarsening is driven by van der Waals forces and there is slippage at the interfacial boundaries between the moving particles and the matrix. The growth rate of the average radius of the particles, R, can be approximately expressed as

where β is a function of the interfacial slippage length b, R, and the average surface distance between the particles h. The parameter b depends on the incompatibility and the degree of entanglement at the interface between the two polymers of the blend. The fact that the coarsening rate is decreased by the presence of a copolymer can be explained by a decrease in the value of b.
Correlation and prediction of crystal solubility and purity
Authors: Teja Amyn S. , Givand Jeffrey C. , Rousseau Ronald W.
Published: April 2004

A thermodynamic model is developed for the correlation and prediction of crystal solubility and purity when lattice substitution by isomorphic or near‐isomorphic impurities is the dominant mechanism that leads to impure crystals. The model is based on earlier observation that the purity of amino acid crystals obtained from aqueous solutions is related to the relative solubilities of the product and impurity in the solvent. A simple relationship is developed that allows the prediction of the purity of crystals obtained from dilute solutions containing both product and impurity species.
Phase equilibrium engineering of supercritical hydrogenation reactors
Authors: Pereda Selva , Bottini Susana B. , Brignole Esteban A.
Published: April 2004

The general problem of phase equilibrium engineering of supercritical hydrogenation reactors is addressed. The conceptual design of process conditions is discussed, as well as procedures to determine the range of feasible operating conditions for two reaction problems: the hydrogenation of vegetable oils and the hydrogenolysis of fatty acid methyl esters. A group contribution equation of state is applied to predict the required phase equilibrium scenarios for solvent containing reactive mixtures.
Absorption of carbon dioxide in aqueous piperazine/methyldiethanolamine
Authors: Bishnoi Sanjay , Rochelle Gary T.
Published: April 2004

Carbon dioxide absorption in 0.6 M piperazine (PZ)/4 M methyldiethanolamine (MDEA) was measured in a wetted wall contactor. The data were simulated using a model that accounts for chemical reactions and transport effects with the eddy diffusivity theory. PZ/MDEA blends absorb CO2 faster than monoethanolamine (MEA) or diethanolamine (DEA) blends with MDEA at similar concentrations. The reaction of PZ to form a monocarbamate is dominant at low loading (<0.14). The reaction of the monocarbamate to form dicarbamate is dominant at high loading. The absorption rate did not follow pseudo first‐order behavior except at very low loading. All carbamate and dicarbamate formation reactions approached instantaneous behavior at high loading. The series resistance due to pseudo first‐order reaction and instantaneous carbamate formation matched data throughout the entire loading range. A typical ammonia plant absorber was simulated at isothermal conditions using the rigorous model. Gas film limitations were dominant (>50%) only at the top of the absorber.
Improved PI controller with delayed or filtered integral mode
Authors: Lee Jietae , Edgar Thomas F.
Published: April 2004

Integral action is almost always included in process control systems to eliminate steady‐state offset without uncertain process gain. The open‐loop pole, however, at the origin of the integral term causes some problems such as integral windup. Various methods to solve these problems were studied. For better control performance and robustness, a filter was added to the integral term, which decouples the effective frequency ranges between the integral and proportional terms without degradation of the integral action. It produces a phase lead in a certain frequency range without having a derivative term, enhancing the control performances and stability robustness. Based on the internal model control method or the direct synthesis method, tuning rules for the proposed controller are given.
Multivariate SPC for startups and grade transitions
Authors: Duchesne Carl , Kourti Theodora , MacGregor John F.
Published: April 2004

Process transitions (grade changeovers, startups, and restarts) are very frequent in industry, and usually lead to the loss of production time, the production of off‐grade materials, and to inconsistent reproducibility of product grades. Two aspects of using multivariate statistical methods based on PCA and PLS to improve process transition performance using historical records of transition data are discussed. First, multivariate SPC approaches are proposed to determine if the process conditions for the commencement of a transition (“startup readiness”) are correct and to assess the successful completion of a transition (“production readiness for the new grade”). The latter is illustrated using a simulated fluidized‐bed process for the production of different grades of linear low‐density polyethylene. Second, analysis tools are suggested for diagnosing the reasons for past transition problems and for monitoring new transitions to ensure repeatable high quality transitions. The latter methods are aimed at reducing the amount of off‐specification materials and reducing transition time, as illustrated on industrial data from restarts of a polymerization process.
Multiplicity of steady states in the codeposition of silicon carbide and carbon
Authors: Kostjuhin Igor M. , Sotirchos Stratis V.
Published: April 2004

Past studies have suggested that the process of chemical vapor deposition of SiC and C from mixtures of methyltrichlorosilane (CH3 SiCl3, MTS), ethylene and hydrogen may exhibit multiple steady states in some regions of its space of operating parameters and conditions. This phenomenon was studied systematically and experimentally in a hot‐wall chemical‐vapor deposition reactor, in which small substrates hung from the sample arm of a sensitive microbalance are used for continuous monitoring of the deposition rate. Multiplicity of steady states occurs in a wide region of the space of process operating parameters, which covers a considerable portion of the range of conditions where deposition of SiC from MTS usually occurs. All cases studied manifest themselves as two stable (observable) deposition states at the same condition, differing by at least one order of magnitude. Various experimental observations and results of the thermochemical equilibrium analysis in the gas phase of the deposition system prove that the appearance of multiplicity in the MTS–C2H4–H2 system manifests the heterogeneous chemistry of the deposition process, not of the homogeneous chemistry or the interactions of the transport and reaction processes. The composition analysis of the deposits reveals that sudden changes of the composition from almost pure carbon at the low deposition rate branch to SiC with excess carbon at the high deposition branch take place at the two limits of the multiplicity region. It indicates that one must operate outside the range of multiple steady states to obtain deposit compositions over the whole range from C to SiC.
Calcined calcium magnesium acetate as a superior SO2 sorbent: I. Thermal decomposition
Authors: Han Dong Hoon , Sohn Hong Yong
Published: April 2004

The reactivity of the calcination product of calcium magnesium acetate (CMA) as a possible dry desulfurization agent was evaluated. The first step in the dry desulfurization reaction—the thermal decomposition of CMA—was investigated by determining the decomposition reaction in three steps: 1. the evaporation of water starting at 65°C, 2. the conversion of CMA into CaCO3 and MgO starting at 275°C, and 3. the decomposition of CaCO3 to CaO starting at 580°C. The pseudo‐first‐order kinetic equation was observed for the CMA decomposition to CaCO3 + MgO, with an activation energy of 165.5 kJ/mol. The decomposition of CaCO3 to CaO also obeyed the pseudo‐first‐order kinetics, with an activation energy of 200 kJ/mol. As discussed in Part II of this series, the calcination product from CMA is quite reactive due mainly to the large porosity and internal surface area generated on the removal of the acetate group during the decomposition reaction. Furthermore, as will be shown in Parts II and III, the sulfation capacity of the calcined CMA is much greater than that of conventional sorbents because its sulfation rate does not level off until complete conversion and its MgO content is also sulfated at temperatures lower than about 900°C.
Simplified design of axial‐flow cyclone mist eliminators
Authors: Brunazzi E. , Paglianti A. , Talamelli A.
Published: April 2004

Mist eliminators have widespread application in many processes, and a large number of different types of mist eliminators are available on the market. Wire‐mesh mist eliminators, vane‐type eliminators, and cyclones are used extensively in many industrial plants. Unfortunately, these separators present some significant drawbacks when they are used in high‐pressure applications or in any application in which a reduction in the diameter of the vessel containing the separator is necessary and high separation efficiency is required. Therefore, over the last several years some important suppliers have developed new axial flow separators (such as Axiflow, Swirl Tube, or Vico‐Spin). Despite the broad range of entrainment removal applications, open literature on this topic is scarce. So far, design of these axial‐flow separators can only be performed by suppliers using proprietary design criteria or using semiempirical equations with an uncertain range of applicability. This work presents new experimental data obtained at atmospheric conditions on three different axial separators and a new design model.