bluuebunny/crossref_metadata_2025 huggingface dataset:
On reaction kinetics for the thermal decomposition of hydrogen sulfide
Authors: Karan Kunal , Mehrotra Anil K. , Behie Leo A.
Published: April 2004

New kinetic data for the hydrogen sulfide decomposition reaction obtained from quartz tubular reactors over a temperature range of 800–1,250°C are reported. The study shows that the overall decomposition reaction has a first‐order dependency on H2S concentrations at these temperatures. The regression of our experimental data gave an overall hydrogen sulfide decomposition reaction rate constant as k (m3/kmol·s) = (1.68±0.86)×1011 exp [(−28,940±840)/T]. Moreover, for the first time, kinetic data from higher temperature (1,500–3,100°C) shock‐tube studies (Olschewski et al., 1994; Woiki and Roth, 1994) and the lower temperature flow reactor study (this work) are reconciled. A single rate constant for H2S decomposition reaction, k (m3/kmol·s) = (1.12±0.11)×1011 exp [−(28,360±200)/T] over a wide temperature range of 800–3,100°C was found to represent satisfactorily the three sets of data.
Coupling of hydrodynamics and chemical reaction in gas‐lift reactors
Authors: Márquez M. A. , Sáez A. E. , Carbonell R. G. , Roberts G. W.
Published: April 2004

A model was developed to study the strong coupling between hydrodynamics and chemical reaction that occurs in external‐loop gas‐lift reactors. The model predicts the liquid circulation rate, as well as the axial profiles of gas holdup, pressure, gas and liquid velocity, and reactant conversion in the riser. The study on the first‐order, irreversible, isothermal reaction in the gas phase nA→B with a change in the number of moles on reaction shows that for n>1, the gas holdup decreases along the riser, the liquid circulation rate is lower than that in the absence of reaction, and liquid circulation decreases as n and k increase. The bubble radius at the sparger and the inlet gas composition can have important effects on reactor performance. Scale‐up strategies that involve increasing the reactor length result in higher reactant conversion, but a lower ratio of liquid circulation rate to gas feed rate.
Effect of small branch inclination on gas–liquid flow separation in T junctions
Authors: Ottens Marcel , Hoefsloot Huub C. J. , Hamersma Peter J.
Published: April 2004

A model developed calculates the mass intake fraction of gas and liquid phase during gas–liquid flow with small liquid holdup values (ϵL ≤ 0.1) through regular T junctions with small branch inclinations. It was derived from the steady‐state macroscopic mechanical energy balance (extended Bernoulli equation) applied to the “inlet‐to‐run” streamline and “inlet‐to‐branch” streamline of both gas and liquid phases. The model results are compared with experimental data of the system air/water–glycerol (0, 33, and 60 wt. %) flowing through a regular (D1 = D2 = D3 = 0.051 m) T junction with branch inclinations ranging from 0° to 0.5°. It was found that the gas–liquid flow split behavior is affected strongly by the liquid viscosity and by branch inclinations of 0.1° and higher. The results predicted with the model agree well with experimental results obtained in a regular T junction with a horizontal inlet run and an inclined side arm.
Bifurcation study of kinetic effects in reactive distillation
Authors: Venimadhavan Ganesh , Malone Michael F. , Doherty Michael F.
Published: April 2004

Simple distillation with multiple chemical reactions of arbitrary stoichiometry is described, with emphasis on effects of chemical kinetics. For low and high Damköhler number, the lightest and heaviest boiling components or azeotropes are known or can be found by known methods. In those limits, the products from a distillation can be anticipated and feasible separation structures can be identified. Feasible structures in these cases are not the same, however, and there are one or more transitions so that different structures can appear for different ranges of the Damköhler number. The transition points correspond to bifurcations, and using bifurcation analysis and continuation methods a systematic method that represents these changes was developed. The practical implication is that the feasibility and product purities for a reactive distillation may depend on production rate, catalyst level, and liquid holdup. The approach is illustrated for the production of methyl‐tert‐butyl ether (MTBE), where a critical Damköhler number for the disappearance of a distillation boundary is found. The influence of pressure on byproduct reactions in methyl acetate synthesis is also studied. A study of the synthesis of isopropyl acetate illustrates how the bifurcation method captures the essential kinetic effects in reactive distillation, including a critical Damköhler for the birth of a reactive azeotrope.
Analysis for the catalytic ignition of methane in a stagnation‐point flow
Authors: Treviño C.
Published: April 2004

The catalytic ignition of a mixture of methane and air in a planar stagnation‐point flow over a platinum foil is studied. The heterogeneous reaction mechanism is modeled with twelve heterogeneous reactions, including the adsorption processes of both reactants, the desorption kinetics of reactants and main products, and a set of surface reactions of the Hinshelwood–Langmuir type. The critical conditions for the catalytic ignition are found using high activation energy asymptotics of the desorption kinetics of the adsorbed reactant O(s). The analytical results are compared with published experimental data. The agreement is good, at least qualitatively.
Solid–liquid equilibria for solutions of binary globular‐protein mixtures
Authors: Hino Toshiaki , Prausnitz John M.
Published: April 2004

A theory of the van der Waals form is used to establish solid–fluid phase diagrams for aqueous solutions containing two kinds of globular proteins. Theory is based on a one‐fluid model that uses a composition‐dependent hard‐sphere diameter and a composition‐dependent solvent‐mediated effective temperature to represent protein solutions. The hard‐sphere reference equation of state is based on the model of Young that correlates computer simulations of fluid–solid coexistence curves for binary hard‐sphere mixtures. The attractive perturbation term uses the inverse‐power potential with variable exponent n. Based on the correlation of George and Wilson and Rosenbaum et al., the optimum range of effective temperature is determined for protein crystallization. For binary protein systems, as the size difference increases, the mutual solubility of unlike proteins declines sharply in the precipitated solid phase. The theoretical results obtained here may be useful for design of protein‐separation processes by crystallization.
Characteristics of microencapsulated PCM slurry as a heat‐transfer fluid
Authors: Yamagishi Yasushi , Takeuchi Hiromi , Pyatenko Alexander T. , Kayukawa Naoyuki
Published: April 2004

The hydrodynamic and heat‐transfer characteristics of slurry containing microencapsulated phase‐change materials (MCPCMs) were investigated experimentally for use as a heat‐transfer fluid. Pressure drop and local convective heat‐transfer coefficients of the slurry flows in a circular tube with uniform heat flux were measured. Slurries consisting of octadecane (C18H38) contained in 2–10‐μm‐dia. microcapsules and pure water were used. The particle volume fractions in the slurry were varied up to 0.3. Results showed that increases in particle volume fractions caused the slurry flow structure to change from turbulent to laminar, and the pressure‐drop reduction of the slurry flow relative to a single‐phase water flow was under the same flow‐rate conditions. The heat‐transfer performance of the slurry also depended on the change in flow structure. When the MCPCMs melted, the local heat‐transfer coefficients for turbulent slurry flows increased relative to those for nonmelting slurry. This phenomenon was influenced by the MCPCM fraction, the degree of turbulence, and the heating rate at the tube wall. The experimental data will be useful in the design of thermal‐energy transportation systems using MCPCM slurry.
Modeling of solid–liquid fluidization in the stokes flow regime using two‐phase flow theory
Authors: Thelen Travis V. , Ramirez W. Fred
Published: April 2004

Two‐phase theory is applied to solid–liquid fluidization in the Stokes flow regime. Because inertial terms can be neglected in the Stokes flow regime, the momentum balances are simplified to an algebraic relation between drag, buoyant, and diffusive forces. The resulting convection‐dispersion model is then applied to an experimental expanded‐bed adsorption system. Two flows are investigated, namely, step changes in the fluidization velocity and step changes in the fluid properties. Experimental data and model simulations are in excellent agreement for both flow configurations. Furthermore, the two‐phase model provides an accurate prediction of the observed time delay in bed expansion for step changes in fluid properties, as well as a mechanistic explanation for its observation.
Optimal operation of multivessel batch distillation columns
Authors: Furlonge H. I. , Pantelides C. C. , Sørensen E.
Published: April 2004

Increased interest in unconventional batch distillation column configurations offers new opportunities for increasing the flexibility and energy efficiency of batch distillation. One configuration of particular interest is the multivessel column, which can be viewed as a generalization of all previously studied batch column configurations. A detailed dynamic model was used for comparing various optimal operating policies for a batch distillation column with two intermediate vessels. A wide variety of degrees of freedom including reflux ratios, product withdrawal rates, heat input to the reboiler, and initial feed distribution were considered. A mixture consisting of methanol, ethanol, n‐propanol and n‐butanol was studied using an objective function relating to the economics of the column operation. Optimizing the initial distribution of the feed among the vessels improved column performance significantly. For some separations, withdrawing product from the vessels into accumulators was better than total reflux operation in terms of energy consumption. Open‐loop optimal operation was also compared to a recently proposed feedback control strategy where the controller parameters are optimized. The energy consumption of a regular column was about twice that of a multivessel column having the same number of stages.
Optimal design of solvent blends for environmental impact minimization
Authors: Buxton Andrew , Livingston Andrew G. , Pistikopoulos Efstratios N.
Published: April 2004

A systematic procedure proposed here selects optimal solvent blends for nonreactive, multicomponent absorption processes accounting for plant‐wide point source environmental interactions. This approach, based on the optimal design technique for pure solvents developed in our earlier work, involves the identification of all agent‐based operations (such as gas absorption and liquid extraction) within the process of interest, the determination of a list of candidate solvent blends satisfying separation task operational and environmental constraints, and the selection of an optimal solvent blend candidate based on global plant‐wide process and environmental constraints. The procedure features new property and operational feasibility constraints, a refined task‐based problem formulation, and a new solution strategy. Two examples are presented to demonstrate the approach, and trade‐offs between cost and environmental impact objectives are discussed.
Improving cubic EOSS near the critical point by a phase‐space cell approximation
Authors: Fornasiero Francesco , Lue Leo , Bertucco Alberto
Published: April 2004

Cubic equations of state (EOSs) are widely used to model the thermodynamic properties of pure fluids and mixtures. However, because they fail to account for the long‐range fluctuations existing in a fluid near the critical point, they do not accurately predict the fluid properties in the critical region. Recently, an approximate renormalization group method was developed that can account for these fluctuations. A similar method is applied to provide corrections to a generalized cubic EOS for pure fluids, which is able to represent all classic cubic EOSs. The proposed approach requires two additional parameters: c̄RG and Δ. The value of c̄RG is correlated to experimental critical compressibility data, while Δ is set equal to 1. The method is applied to predict the saturated liquid density of fluids of different polarity, and the corrections to the original EOS are found to significantly improve the predictions of this property both far from and close to the critical point. Finally, a correlation is presented for the direct evaluation of the parameter c̄RG from the value of the critical compressibility factor.
Design of a simulated moving bed in the presence of mass‐transfer resistances
Authors: Azevedo Diana C. S. , Rodrigues Alírio E.
Published: April 2004

Two different strategies presented determine the sets of operating conditions that yield desired product purities for an SMB separator. The search was performed numerically by solving the analogous true‐moving‐bed model equations. A linear equilibrium binary mixture and mass‐transfer effects were taken into account by using a bilinear driving force approximation to describe intraparticle diffusion. The strategies are equivalent and produce the same results. Comparison with the linear equilibrium model strategy indicates that there are new limiting values for the flow‐rate constraints in the presence of mass‐transfer effects. The constraints on sections I, II and III were more restrictive than those derived from equilibrium models, whereas the constraint on section IV was less affected. Qualitative observations drawn from a model of intermediate complexity were also presented to confirm our results.
Emulsion breakdown: Mechanisms and development of multilayer membrane
Authors: Kawakatsu Takahiro , Boom Remko M. , Nabetani Hiroshi , Kikuchi Yuji , Nakajima Mitsutoshi
Published: April 2004

Phase inversion phenomena of oil‐in‐water (triolein in water‐containing sodium lauryl sulfate, SDS) emulsions permeating through hydrophobic micropores were visualized using an optically accessible array of well‐defined microchannels and a microscope video system. Oil droplets coalesced on the hydrophobic microchannel surface and permeated to form a continuous oil layer at the outlet. Water was repelled from the surface, and large, irregular‐sized water droplets were created at the outlet. To evaluate the emulsion breakdown mechanism practically, the emulsions were filtered with hydrophobic polytetrafluoroethylene membranes of several pore sizes. In all cases, the permeate was separated into two phases, an oily and an aqueous one. The three pore sizes, 1‐, 5‐ and 10‐μm membranes, were stacked into a layer, and with the multilayer membrane the breakdown ability of the 1‐μm membrane was effectively combined with the oil droplet growth ability of the 5‐ and 10‐μm membranes.
Structural design for on‐line process optimization: II. Application to a simulated FCC
Authors: Loeblein C. , Perkins J. D.
Published: April 2004

A previously proposed methodology for analyzing the economic performance of different structures of an integrated model predictive regulatory control and on‐line optimization system is illustrated using the simulated case study of a fluid‐catalytic cracker. The economic performance of two previously proposed regulatory control structures, the conventional and riser‐regenerator control structures, is assessed both in isolation and as part of a real‐time optimization system. The results show that the economic performance of the conventional structure is superior when operated in isolation. The theory, however, predicts that an optimally configured real‐time optimization system based on the riser‐regenerator structure will deliver superior performance overall. The predictions of the method were confirmed using Monte Carlo simulations.
Kinetic and thermodynamic analysis of liquid‐phase benzene hydrogenation
Authors: Singh Utpal K. , Vannice M. Albert
Published: April 2004

Liquid‐phase benzene hydrogenation on Pd/η‐Al2O3 catalysts was studied between 358 and 488 K and over an H2 pressure range of 7 to 54 atm. Kinetic data were obtained under differential reaction conditions that were free of all transport limitations, thus allowing the development of a kinetic model that could be compared to that for the vapor‐phase reaction. In contrast to vapor‐phase studies at 1 atm, no activity maximum was observed during liquid‐phase hydrogenation between 358 and 493 K and at 54‐atm H2 pressure. A model that invokes dissociative, noncompetitive H2 adsorption, addition of the first H atom as the rate‐determining step, and a concurrent dehydrogenated benzene surface species was consistent with experimental observations. The fitted parameters were thermodynamically consistent and agreed with those from previous vapor‐phase studies. In addition, thermodynamic analysis of the quasi‐equilibrated hydrogen adsorption process indicates that in the presence of solvent effects, increasing hydrogen solubility in the liquid phase can increase the surface coverage of hydrogen. In the absence of any solvent effect, however, surface coverage depends only on the partial pressure of hydrogen and is independent of the nature of the solvent. Furthermore, when solvent effects exist, use of the liquid‐phase hydrogen concentration in the rate expression yields a solvent‐independent adsorption equilibrium constant, while in the absence of solvent effects, P should be used in the rate expression to acquire this parameter.
Modeling the sorption of toxic metals on chelant‐impregnated adsorbent
Authors: Ravindran Varadarajan , Stevens Mario R. , Badriyha Badri N. , Pirbazari Massoud
Published: April 2004

The application of a microporous adsorbent, such as activated carbon impregnated with 8‐hydroxyquinoline (oxine), a chelating agent, to remove toxic metals from the aqueous phase is studied. A pore and surface diffusion model for predicting the dynamics of fixed‐bed adsorbers using oxine‐impregnated carbon is also discussed. The model incorporates adsorption equilibrium and kinetic parameters determined from independent isotherm tests and rate experiments, as well as adsorber column flow characteristics. Three modeling scenarios were employed with reference to intraparticle transport: (a) combined pore diffusion and surface diffusion; (b) pore diffusion alone, suppressing the effect of surface diffusion; and (c) surface diffusion alone, suppressing the effect of pore diffusion. Surface diffusion alone provided reasonably good predictions of the adsorber dynamics, as reflected by results from adsorber experiments. It is postulated that the sorption mechanism could be a combination of film transport of metal ions, followed by surface diffusion into adsorbent particles, and subsequent chelation of metal ions with the oxine molecules sorbed on carbon. The sorbed metals could be completely recovered under acidic conditions during the carbon regeneration process.
Gasification of spent alkylation sulfuric acid droplets in hot environments
Authors: Wang Difei F. , Shaw Benjamin D. , Yang Albert
Published: April 2004

Experimental results on gasification behaviors of freely‐falling spent alkylation sulfuric acid droplets in high‐temperature environments are presented, as well as baseline data on evaporation of water droplets and sulfuric acid droplets. Low evaporation constants have been measured for sulfuric acid droplets and spent acid droplets (relative to water droplets). Shape distortions and fragmentation (microexplosions) of spent acid droplets were also observed, and it has been found that initial droplet diameters are important in determining onset times and intensities of microexplosive behaviors. Simplified theory is developed, which suggests that the droplet distortion and fragmentation behaviors may be a result of SO2 bubble formation inside the droplets.
Tailor‐made asymmetric PVDF hollow fibers for soluble gas removal
Authors: Li K. , Kong J. F. , Wang Dongliang , Teo W. K.
Published: April 2004

Tailor‐made polyvinylidene fluoride (PVDF) asymmetric hollow‐fiber membranes and their membrane modules were employed for soluble gas removal, such as H2S from waste gas streams. This study focused on the techniques of fabricating and characterizing the PVDF asymmetric hollow‐fiber membranes and their membrane modules for removal of H2S using an aqueous solution containing 10% NaOH. A laminar parabolic velocity profile was used to characterize the flow of the H2S gas mixture in the hollow‐fiber lumen. Effects of operating conditions and the morphological structures of the membranes on the membrane´s coefficient, kAM, were examined both theoretically and experimentally. The capabilities of the hollow‐fiber membranes developed for removal of H2S from waste gas streams were evaluated and compared with conventional symmetric hydrophobic hollow‐fiber membranes, such as polypropylene. An analysis of H2S transfer across the more developed PVDF membranes reveals that the membrane´s coefficient, kAM, evaluated from its structure parameters, such as the effective surface porosity and mean radius, agreed well with the experimental data obtained from absorption experiments.
Gas permeation properties of ion‐exchanged faujasite‐type zeolite membranes
Authors: Kusakabe K. , Kuroda T. , Uchino K. , Hasegawa Y. , Morooka S.
Published: April 2004

NaY‐type zeolite membranes were synthesized on a porous support tube by a hydrothermal process. The membranes were ion‐exchanged with Li+ and K+ ions, and permeances through the membranes were determined for an equimolar mixture of CO2 and N2, as well as for single‐components thereof, at a temperature range of 0–400°C. The permeance to CO2 showed a maximum at 100°C, but CO2/N2 selectivity decreased with increasing temperature. The zeolite membranes that were exchanged with K+ and Li+ ions gave higher and lower CO2/N2 selectivities, respectively, than were found for the NaY‐type membrane. The permeation properties of the ion‐exchanged zeolite membranes were analyzed using a sorption–diffusion model. The high CO2/N2 selectivity of the K‐exchanged membranes can be explained by the decrease in N2 sorptivity for the mixed feed.
Global optimization of a dryer by using neural networks and genetic algorithms
Authors: Hugget A. , Sébastian P. , Nadeau J.‐P.
Published: April 2004

For many optimum design problems, the objective function is the result of a complex numerical code and may not be differentiable and explicit. The first aim is to propose a way of solving such complexity on an example problem. A novel and global strategy involving artificial neural networks and a genetic algorithm is presented and validated for an industrial convective dryer. To begin with, a method to represent a drying model using artificial neural networks is defined. This method is tested and the results are compared with those obtained with classic numerical methods. This approach drastically reduces simulation times and maintains good accuracy and generalization properties. Second, the associated optimal design problem is considered. This optimization appears as a difficult combinatorial problem with a complex objective function that involves different economical criteria. The second aim is to present the methodology to solve this problem using genetic algorithms. Final results illustrate the efficiency of this global approach.
Heat exchanger network retrofit considering pressure drop and heat‐transfer enhancement
Authors: Nie X. R. , Zhu X. X.
Published: April 2004

For the heat‐exchanger‐network (HEN) retrofit project to be successful it should satisfy not only a specified heat‐recovery target but also pressure‐drop constraints. The problem of dealing with the pressure‐drop aspects is very complex, since there are many options that affect pressure drop. To integrate these options more systematically and solve this problem more effectively, a decomposition strategy is proposed. At the first stage, the unit‐based model is used to indicate which units require additional area. In the second stage, special attention is paid to these units, where area distribution, shell arrangements, the use of heat‐transfer enhancement, and other options are optimized for these units. At the same time, the units without additional area requirement are modeled using simple models. Thus units with and without additional area requirements are treated differently during optimization. By doing this, the pressure drop can be calculated accurately while the overall model remains simple and easy to solve. Heat‐transfer enhancement is a very attractive option for HEN retrofit, since it can effectively eliminate or reduce the additional surface area space. This will eliminate additional piping, new shells or units, and their required space, which lead to much lower modification costs and less implementation time.
Hierarchical procedure for plantwide control system synthesis
Authors: Zheng Alex , Mahajanam Rama V. , Douglas J. M.
Published: April 2004

A hierarchical procedure for synthesizing an optimal plantwide control system is proposed for an existing continuous chemical process. Alternative plantwide control systems are synthesized and are compared based on economics. The cost associated with dynamic controllability is quantified by the controllability index ν introduced by Zheng and Mahajanam (1999). The procedure is illustrated on a simple reactor‐separator‐recycle system. Although the procedure is discussed for an existing plant, a simple modification of the approach can be used to determine the optimum surge capacities of a process, including reflux drums and column sumps—a problem of increasing interest.
Steady states and constraints in model predictive control
Authors: Rao Christopher V. , Rawlings James B.
Published: April 2004

Studies on the theory of model predictive control include the assumption that the origin is in the interior of the feasible region (that is, the inequality constraints are not active at steady state). The reason for making this assumption is that without it one cannot guarantee feasibility of the control problem on the infinite horizon because of the finite horizon parameterization of the input with an unconstrained linear feedback law. As demonstrated in this article, however, this assumption often does not hold in practice. A strategy for handling inequality constraints active at steady state is presented by projecting the system onto the active constraints under the finite horizon parameterization of the input, as well as an algorithm for constructing the optimal linear feedback law that constrains the system to the active constraints. Feasibility is obtained using output admissible sets. For the steady‐state target calculation, we propose an algorithm utilizing exact penalties that treats systems in a unified fashion with more inputs than outputs and vice versa. Assuming the system is detectable, it is proven that the algorithm yields a unique steady‐state target.
Nonlinear control of particulate processes
Authors: Chiu Timothy , Christofides Panagiotis D.
Published: April 2004

A general methodology is proposed for the synthesis of practically‐implementable nonlinear output feedback controllers for spatially‐homogeneous particulate processes modeled by population balance equations. Initially, a nonlinear model reduction procedure based on a combination of the method of weighted residuals and the concept of approximate inertial manifold is presented for the construction of low‐order ordinary differential equation (ODE) systems that accurately reproduce the dominant dynamics of the particulate process. These ODE systems are then used for the synthesis of nonlinear low‐order output feedback controllers that enforce exponential stability in the closed‐loop system and achieve particle‐size distributions with desired characteristics. Precise closed‐loop stability conditions are given and controller implementation issues are discussed. The proposed nonlinear control method is successfully applied to a continuous crystallizer, and is shown to outperform a proportional‐integral controller and cope effectively with model uncertainty and measurement delays.
Three‐dimensional developing flow model for photocatalytic monolith reactors
Authors: Hossain Md. Moazzem , Raupp Gregory B. , Hay Steven O. , Obee Timothy N.
Published: April 2004

A first‐principles mathematical model describes performance of a titania‐coated honeycomb monolith photocatalytic oxidation (PCO) reactor for air purification. The single‐channel, 3‐D convection–diffusion–reaction model assumes steady‐state operation, negligible axial dispersion, and negligible homogeneous reaction. The reactor model accounts rigorously for entrance effects arising from the developing fluid‐flow field and uses a previously developed first‐principles radiation‐field submodel for the UV flux profile down the monolith length. The model requires specification of an intrinsic photocatalytic reaction rate dependent on local UV light intensity and local reactant concentration, and uses reaction‐rate expressions and kinetic parameters determined independently using a flat‐plate reactor. Model predictions matched experimental pilot‐scale formaldehyde conversion measurements for a range of inlet formaldehyde concentrations, air humidity levels, monolith lengths, and for various monolith/lamp‐bank configurations. This agreement was realized without benefit of any adjustable photocatalytic reactor model parameters, radiation‐field submodel parameters, or kinetic submodel parameters. The model tends to systematically overpredict toluene conversion data by about 33%, which falls within the accepted limits of experimental kinetic parameter accuracy. With further validation, the model could be used in PCO reactor design and to develop quantitative energy utilization metrics.
Diagnosis and optimization approach for heat exchanger network retrofit
Authors: Zhu X. X. , Asante N. D. K.
Published: April 2004

A new modeling approach for HEN retrofit design is discussed. The design task consists of a search for topology changes, called the diagnostic stage, followed by an evaluation stage and a cost optimization stage. Promising modifications are selected from the diagnosis stage and assessed in terms of the impacts on implementation cost, operability, and safety. The options deemed impractical are removed and the remaining ones are optimized together with the existing HEN to give the final HEN retrofit design. The new approach combines mathematical optimization techniques with a better understanding of the retrofit problem, based on thermodynamic analysis and practical engineering, to produce a systematic procedure capable of efficiently solving industrial‐size retrofit problems. The network pinch concept provides new insights to the HEN retrofit problem and plays an important role in selecting promising modifications, forming the foundation of the new method. This concept, when applied to mathematical formulation, significantly simplified the mathematical models while maintaining good quality of solutions. This approach allows the design tasks to be automated with user interactions.
Monitoring and diagnosing batch processes with multiway covariates regression models
Authors: Boqué Ricard , Smilde Age K.
Published: April 2004

Multivariate statistical procedures for monitoring the behavior of batch processes are presented. A new type of regression, called multiway covariates regression, is used to find the relationship between the process variables and the quality variables of the final product. The three‐way structure of the batch process data is modeled by means of a Tucker3 or a PARAFAC model. The only information needed is a historical data set of past successful batches. Subsequent new batches can be monitored using multivariate statistical process control charts. In this way the progress of the new batch can be tracked and possible faults can be easily detected. Further detailed information from the process can be obtained by interrogating the underlying model. Diagnostic tools, such as contribution plots of each of the variables to the observed deviation, are also developed. Finally, on‐line predictions of the final quality variables can be monitored, providing an additional tool to see whether a particular batch will produce an out‐of‐spec product. These ideas are illustrated using simulated and real data of a batch polymerization reaction.
Gas‐phase polymerization of propylene with a highly active ziegler‐natta catalyst
Authors: Samson Job Jan C. , van Middelkoop Bart , Weickert Günter , Westerterp K. Roel
Published: April 2004

The polymerization of propylene in the gas phase has been studied with the same high‐activity catalyst as was used for liquid‐phase polymerizations in earlier work. Catalyst injection, the influence of the support bed, and precontacting of the catalyst with the cocatalyst and the electron donor have been investigated. Furthermore, different ways of prepolymerization were tested. The kinetics can be described with the same model as was used for the liquid‐phase polymerizations. The reaction rate was proportional to the pressure at pressures above 5 bar; at lower pressures the rates are lower. Comparison of the reaction rates in the gas phase and liquid phase showed the rates in the gas phase were still lower than in the liquid phase. The fast decay observed in the gas‐phase polymerizations indicated that thermal runaway and deactivation may occur at temperatures above 40–45°C.
Gas separation properties of organosilicon plasma polymerized membranes
Authors: Roualdes Stéphanie , Van der Lee Arie , Berjoan René , Sanchez José , Durand Jean
Published: April 2004

Thin films were polymerized from different organosilicon compounds in a radio‐frequency plasma deposition process. The properties of the layers were characterized with respect to the deposition rate, the density, the refractive index, and the chemical structure determined by FTIR and XPS analysis. The qualification of the films for gas‐selective membranes was tested on different porous substrates using N2, H2, O2, CO2, and CH4. Both structure and permeation performances of the synthesized films were correlated with the composite plasma parameter V/F·M (V: input voltage; F: monomer flow rate; M: monomer molecular weight). At low V/F·M ratio, the thin layers are mainly constituted of the [(CH3
)2–Si(–O)2] environment (monomer and polydimethylsiloxane one). Increasing the V/F·M results in a more “inorganic” chemical structure, higher O/Si ratio, refractive index, and density, the materials tend toward a silicalike structure. Concurrently, the prepared membranes have solution–diffusion‐controlled or Knudsen‐like separation factors, depending on whether plasma conditions are soft or hard.
Neural‐network modeling and optimization of induced foreign protein production
Authors: Tholudur Arun , Ramirez W. Fred
Published: April 2004

An experimental verification and validation of the neural network parameter function approach to modeling dynamic systems is provided. The neural‐network parameter‐function modeling scheme utilizes some a priori process knowledge (usually material balances) and experimental data to develop a dynamic neural‐network model. Other models based on fundamental principles are also developed. The experimental system under consideration is the host‐vector system Escherichia coli D1210 and plasmid pSD8, which produces the foreign protein β‐galactosidase under the effect of the inducer IPTG. Optimal operational conditions are derived and the neural‐network‐based model is shown to better predict the dynamics and optimum for protein production than the proposed fundamental kinetic models.
Synthesis approach to the determination of optimal waste blends under uncertainty
Authors: Chaudhuri Prosenjit , Diwekar Urmila
Published: April 2004

The generalized approach to the problem of synthesis under uncertainty is to formulate it as a stochastic optimization problem that involves optimization of a probabilistic function obtained by sampling over uncertain variables. The computational burden of this approach can be extreme and depends on the sample size used for characterizing the parametric uncertainties. A new and efficient approach for stochastic process synthesis is presented. The goals are achieved through an improved understanding of the sampling phenomena based on the concepts derived from fractal geometry. A new algorithm for stochastic optimization based on these concepts to accelerate the process of synthesis under uncertainty is presented. Apart from the benchmark HDA synthesis problem, a real‐world problem of synthesizing optimal waste blends is analyzed to test the applicability of this novel approach in addressing the general problem of synthesis under uncertainty. The solution of this real‐world large‐scale synthesis problem is presented under uncertainty through the application of the new stochastic annealing algorithm, which takes into consideration novel sampling methods used in probabilistic analysis of process models.
Measurement of the filtration diffusivity D(ϕ) of a flocculated suspension
Authors: Landman Kerry A. , Stankovich J. M. , White Lee R.
Published: April 2004

In modeling the pressure filtration of flocculated suspensions using compressional rheology, the filtration diffusivity function D(ϕ) plays the role of a diffusion coefficient in determining the time scale of the filtration process. Its dependence on volume fraction is an important factor in filtration process design. The volume of filtrate expressed per unit membrane is V = βt1/2. The value of β depends on D(ϕ) and various relationships between the two that involve data differentiation have been discussed in the literature. These involve an unknown but bounded parameter. Here a physical approximation is made that justifies setting this parameter to zero. Further, a new approximate expression for β2 in terms of D(ϕ) that avoids data differentiation is determined. Alternatively, the hindered settling factor can be recovered from an analytic expression involving data differentiation of β data with applied pressure. The accuracy of each of these approximations is assessed.
Dynamic rate‐based model for multicomponent batch distillation
Authors: Kreul L. U. , Górak A. , Barton P. I.
Published: April 2004

Dynamic rate‐based approach for modeling and simulating batch distillation is presented. To predict separation efficiencies, mass transfer was considered explicitly using the Maxwell‐Stefan. Process dynamics were modeled by considering all relevant dynamic changes in the system, including the column periphery. The model, implemented at the large‐scale ABACUSS modeling environment, addresses implications of the dynamic and multicomponent nature of the process for its mathematical representation, the use of mass‐transfer coefficients, and the need for further experimental correlations. To validate the model, a series of separations of the highly nonideal quaternary system of methyl acetate, methanol, acetic acid, and water in a pilot‐plant batch‐distillation column with structured packings were conducted. The results show that a rate‐based approach can predict the column operation within the experimental error over the entire operation time without fitting of any kind. For the first time the use of a rate‐based approach on the ground of the Maxwell‐Stefan equations for a multicomponent batch‐distillation process was validated experimentally. Comparison of predictions of the rate‐based approach with simulations based on an equally tailored equilibrium‐stage model shows that the largest differences between the two approaches occur in short periods of very significant changes in the column profile. The additional effort of rate‐based modeling can be justified for the design of operating policies of multicomponent batch‐distillation processes.
Catalyst attrition in fluidized‐bed systems
Authors: Werther J. , Reppenhagen J.
Published: April 2004

Catalyst attrition has often been a major obstacle in the development of new fluidized‐bed processes and it is still of concern for existing processes whenever the catalyst is changed. The main consequence of attrition is the generation of fines and the resulting loss of valuable material. In this work the attrition of FCC catalysts in a fluidized‐bed system consisting of a bubbling bed, cyclone gas cleaning, and external solids recirculation is studied. An approach developed describes the attrition‐induced overall solids loss based on the main attrition sources: grid jet attrition, bubble‐induced attrition, and attrition in the cyclone. Their respective contributions to the overall attrition rate are calculated from separately derived model equations that take into account the design of the system, the operating conditions, and the material properties of the catalysts. The solids entrainment from the bed into the cyclone is shown to have a strong effect on the attrition‐induced loss rate and must therefore be taken into account for an a‐priori prediction.
Modeling agglomeration processes in fluid‐bed granulation
Authors: Cryer Steven A.
Published: April 2004

Many agrochemicals are formulated as water dispersive granules through agglomeration, beginning with a fine powder (∼1 μm) and ending with granules on the order of 500 μm. Powders are charged into a granulation system with a liquid binding agent, and granules are subsequently grown to an appropriate size. Granulation in fluid beds is presented using a mass conserving discretized population balance equation. Coalesce kernels governing the rate and extent of granulation are assumed dependent on the Stokes number, which is indirectly linked to important process variables (air and binder flow rate, bed charge, bed geometry) such that the physical processes governing particle coalescence and rebound are correlated to process variables. A new coalescence kernel is proposed based on physical insight, simplicity, and deterministic equivalent modeling to account for uncertainty. This kernel is based on a Stokes number method where uncertainty in the Stokes number is characterized by polynomial chaos expansions. The magnitude of the coalescence kernel is proportional to the probability of the distribution of Stokes number exceeding a critical value. This mechanistic/semiempirical approach to fluid‐bed agglomeration fosters an environment for process scaleup by eliminating specific equipment and process variable constraints to focus on the underlying mechanisms for proper scale‐up procedures. Model predictions using this new kernel are then compared to experimental pilot‐plant observations.
Modeling of particle evolution in aerosol reactors with coflowing gaseous reactants
Authors: Bensberg Andreas , Roth Paul , Brink Reinhard , Lange Helmut
Published: April 2004

A simple model is presented for simultaneous nucleation and coagulation in combination with convective and diffusive particle transport in gas‐phase aerosol reactors over the entire particle‐size spectrum. This model is applied to a spatially inhomogeneous aerosol reactor. The flow and reaction characteristics correspond to a confined coflow diffusion flame of SiCl4 and NH3 in a wall‐heated flow. The reactor geometry and the initial and boundary conditions suggest the application of the boundary layer approximations. The reaction process is described by the flame‐sheet model. The particle‐size spectrum is approximated by a unimodal lognormal function. The characteristics of the SiN4 aerosol (concentration, polydispersity, and average particle size) calculated at every point in the reactor vary significantly in space. At the position of the reaction zone in the outlet cross section, for example, the average particle radius is small (rg ≈1 nm) and the standard deviation is large (σ > 2). At other radial positions the particles are much larger (rg
≈15 nm) and the size distribution is almost self‐preserving (σ≈1.36).
Situating the high‐density circulating fluidized bed
Authors: Grace John R. , Issangya Allan S. , Bai Dingrong , Bi Hsiaotao , Zhu Jingxu
Published: April 2004

Gas–solid flow in high‐density circulating fluidized‐bed risers differs significantly from that in conventional flow regimes, including fast fluidization and pneumatic conveying. It is shown that when there is no downward flux at the wall at high suspension densities, and hence no core/annulus structure, the flow corresponds to a dense suspension upflow regime. This flow regime is explained, and its key properties given. A lower boundary is also proposed for this new flow regime based on available data.
Novel mixing tank for improved gas–liquid mass transfer and solids suspension
Authors: Palmer Harvey J. , Hughes Christine M. C. , Lessen Martin , Greenleaf Jennifer , Buehler Kristen L. , Villalobos Carmen
Published: April 2004

A novel, baffleless, mixing tank configuration presented provides greater mixing efficiency than conventional technologies under a wide variety of operating conditions. The concept uses a cylindrical tank with a contoured bottom and a shroud that surrounds an axial flow impeller and supports a set of stationary “antiswirl” fins positioned in close proximity to the impeller blades. The angle and pitch of the fins impart an angular component to the flow that is equal and opposite to the swirl induced by impeller rotation, thereby eliminating the need for wall baffles. The contoured bottom eliminates dead spaces beneath the impeller and in the corners of flat‐bottom mixing tanks. This alternative mixing system, named the ASSET system, is compared to conventional mixing configurations through experiments of power consumption, gas–liquid mass transfer, and solids suspension. Measurements of Power number vs. Reynolds number reveal that it has power requirements similar to those of high‐efficiency axial‐flow turbines in a conventional tank geometry. Unsteady‐state oxygen absorption and stripping experiments are used to determine the interphase mass‐transfer coefficient as a function of power input for each configuration at superficial gas feed velocities of 0.057 to 0.17 cm/s. With this system, KL a values are on average 30 to 50% greater than those obtained in a conventional mixing tank at the same power input. Measurements of the height of the well‐mixed region of suspended solids in a 15 wt. % mixture of 0.1‐cm‐dia. glass beads in water indicate that the ASSET system requires ∼30% less power to achieve equivalent extents of solids suspension than the conventional setup.
Modeling of liquid/liquid separation by pervaporation: Toluene from water
Authors: Meuleman Erik E. B. , Bosch Bert , Mulder Marcel H. V. , Strathmann Heiner
Published: April 2004

The resistances‐in‐series model, the modified solution‐diffusion model, the Flory–Rehner theory, and the film theory were used to calculate the diffusion coefficients of two components of a liquid‐feed mixture that are separated by pervaporation. The toluene and water fluxes through EPDM membranes of various thicknesses were modeled for different mass‐transfer coefficients in the feed boundary layer (kL). It is shown that the toluene flux depends strongly on the kL and the membrane resistance affects significantly the toluene flux only for very high values of kL (>10−4 m/s). The water flux increases linearly with the reciprocal value of the membrane thickness and is not affected by the boundary‐layer resistance. The small increase in the water flux, due to the toluene concentration increase in the feed, can adequately be described by this model.