bluuebunny/crossref_metadata_2025 huggingface dataset:
Interfacial tension reduction and coalescence suppression in compatibilized polymer blends
Authors: Lepers Jean‐Christophe , Favis Basil D.
Published: April 2004

This work examines the influence of well‐defined diblock copolymers on the interfacial tension and morphology of an immiscible polymer blend. The system studied is a blend of 80% polystyrene and 20% ethylene propylene copolymer compatibilized by diblock copolymers of poly(styrene‐hydrogenated butadiene). The diblocks differed in that one contained 53% styrene (symmetric diblock) and the other 30% styrene(asymmetric diblock). The interfacial tension was performed using the breaking‐thread technique, and the results were compared to both the theoretical predictions of Noolandi and Hong, and Leibler. The morphology was tracked using an emulsification curve, comparison of which with the interfacial tension results made it possible to estimate the relative role of interfacial tension and coalescence reduction in particle‐size reduction phenomena.
Modeling of multicomponent drying of polymer films
Authors: Alsoy Sacide , Duda J. Larry
Published: April 2004

The drying of volatile solvents from a coated film is a complicated process since it involves simultaneous heat and mass transfer controlled by complex transport and thermodynamic behavior of polymer solutions. In this work, a model is developed to describe the drying behavior of multicomponent polymer solutions deposited on the impermeable substrate. A key component of the model is incorporation of multicomponent diffusion coefficients that consist of thermodynamic factors and self‐diffusivities. Vrentas‐Duda free‐volume theory was used for predicting concentration and temperature dependency of self‐diffusion coefficients. Drying processes in ovens with different zones in which temperature, velocity, and/or partial pressure of each solvent vary from zone to zone are considered. The predictions from the models provide detailed information about the relative importance of internal and external resistances to drying, effect of operating conditions, effect of the multiple‐zone approach, as well as sensitivity of all physical properties on the drying rate. The validity of the model was confirmed by comparing predictions with the drying data available in the literature and the data collected in our laboratories. The experimental data and the simulation results are in good agreement.
Surfactant and viscoelastic effects on drop deformation in 2‐D extensional flow
Authors: Tretheway Derek C. , Leal L. Gary
Published: April 2004

The deformation and breakup of polymeric drops suspended in an immiscible Newtonian fluid are examined for planar extensional flow in a computer controlled four roll mill are examined. Previous work from this lab with aqueous polymeric drops showed an increase in the degree of deformation compared to Newtonian drops, and two distinct breakup mechanisms, tip streaming, and tip streaming with stretch. This led to the conclusion that breakup for non‐Newtonian drops occurred at a lower capillary number and by a distinctly different mechanism than for a Newtonian drop with a similar ratio of internal to external viscosity. This conclusion was puzzling, because numerical computations with the Chilcott‐Rallison model showed that the influence of viscoelasticity for that model was to decrease the degree of deformation rather than increasing it. In this work, the results for aqueous polymer solutions are compared with results for ethylene glycol polymer solutions. The results suggest that tip streaming and tip streaming with stretch in the original experiments resulted from an undetected surfactant effect, which acts to produce a more pointed drop shape that destabilizes the drop to tip streaming. Viscoelasticity, on the other hand, acts to decrease the deformation of polymeric drops at a given capillary number and increases the critical capillary number, as predicted numerically, when compared to Newtonian drops with an equivalent viscosity ratio.
Measurements of solid concentration in a downward vertical gas–solid flow
Authors: Schiewe T. , Wirth K. E. , Molerus O. , Tuzla K. , Sharma A. K. , Chen J. C.
Published: April 2004

New results from experiments performed in a 15‐cm‐diameter downflow fast‐fluidized bed are presented. Tests were conducted at room temperature and near atmospheric pressure, with 125‐μm glass beads. Superficial gas velocities range from 0 to 6.6 m/s. Two different measurement techniques—gamma‐absorption tomography and capacitance sensing—were applied to the gas–solids flow in the downer tube. The average local solid fractions from both measurement techniques are compared for various operating conditions of the gas–solid flow. In general, good agreement was obtained between the solid concentration measurements from both measurement techniques. It is demonstrated that combined use of both measurement techniques offers the best chance to get time‐average information about the concentration distribution over the whole cross section.
Binary permeation through a silicalite‐1 membrane
Authors: van den Broeke Leo J. P. , Bakker Wridzer J. W. , Kapteijn Freek , Moulijn Jacob A.
Published: April 2004

Permeation of binary gaseous mixtures through a silicalite‐1 membrane was studied. Results are reported for the fluxes and separation factors as a function of the feed composition, the pressure and the temperature. For the various cases a comparison is made between the binary and the single‐component permeation behavior. For the permeation of the mixtures the stronger adsorbed component is little affected by the presence of the weaker adsorbed component. On the other hand, for the weaker adsorbed component a clear reduction in the flux is observed. As a consequence, the separation factor for mixtures differs considerably from the so‐called ideal separation factor, which is the ratio of the one‐component permeances. As expected, the separation factor depends on the temperature. The separation factor is also a function of the composition and the feed pressure. This means that the binary equilibrium adsorption cannot be described with the extended Langmuir model. Binary permeation and separation behavior is described using the ideal adsorption solution theory for equilibrium adsorption.
Impact of gas composition on natural gas storage by adsorption
Authors: Mota José P. B.
Published: April 2004

Adsorption storage is the most promising low‐pressure alternative for storing natural gas, but some operational difficulties hinder the success of this technology. From a modeling perspective, this article addresses the impact of gas composition on the cyclic behavior of adsorptive natural gas storage systems. The cyclic operation of an onboard storage reservoir is modeled as a series of consecutive two‐step processes, each consisting of charge with a fixed composition gas mixture followed by discharge at constant molar flow rate. Special attention is given to the composition and calorific value of the gas delivered by the storage system and to performance quantifiers that measure net deliverable capacity as a function of cycle number. Evidence of a cyclic steady state is given, and the numerical results are compared with experimental measurements. The results show the need for identifying and evaluating economical means of removing the higher molecular‐weight hydrocarbons and other highly adsorbed species from the gas stream before charging the storage reservoir.
Optimal synthesis of refrigeration cycles and selection of refrigerants
Authors: Vaidyaraman Shankar , Maranas Costas D.
Published: April 2004

The optimal synthesis of the refrigeration configuration and the selection of the best refrigerants that satisfy a set of process cooling duties at different temperatures is addressed. This approach simultaneously selects refrigerants and synthesizes refrigeration structures by minimizing a weighted sum of investment and operating costs. A superstructure representation considers the majority of refrigeration cycle features encountered in real complex multistage refrigeration cycles such as economizers, multiple refrigerants, and heat integration. A novel theoretical treatment of modeling representations and algorithmic improvements is introduced. Results, for example, involving multiple refrigerants, cooling loads, and heat sinks are obtained. Complex, nonintuitive topologies typically emerge as the optimal refrigeration configurations that are better than those obtained when refrigeration synthesis is performed after refrigerant selection.
Structural design for on‐line process optimization: I. Dynamic economics of MPC
Authors: Loeblein C. , Perkins J. D.
Published: April 2004

The structural design of integrated online process optimization and regulatory control systems based on an economic analysis of different structures is addressed. The regulatory control layer is assumed to be implemented using model predictive control (MPC) techniques. An approach to the analysis of the dynamic economics of MPC is presented which uses the state‐space formulation as the plant model. Output feedback is performed in the framework of linear quadratic filtering theory using a Kalman filter. Using the unconstrained model predictive control law, the variance of the constrained variables of the closed‐loop system subject to stochastic disturbances is analyzed. Based on the variance of the constrained variables, the amount of necessary backoff from the constraints due to regulatory disturbances is calculated and the dynamic economics are established. The dynamic economics of the model predictive regulatory control system are incorporated into the method of the average deviation from optimum analyzing the economic performance of an online optimization system. Thus, different structures of the integrated system of online optimization and MPC‐based regulatory control can be analyzed in terms of their economic performance, and the necessary structural design decisions can be taken.
On‐line multiscale filtering of random and gross errors without process models
Authors: Nounou Mohamed N. , Bakshi Bhavik R.
Published: April 2004

Data Rectification by univariate filtering is popular for processes lacking an accurate model. Linear filters are most popular for online filtering; however, they are single‐scale best suited for rectifying data containing features and noise that are at the same resolution in time and frequency. Consequently, for multiscale data, linear filters are forced to trade off the extent of noise removal with the accuracy of the features retained. In contrast, nonlinear filtering methods, such as FMH and wavelet thresholding, are multiscale, but they cannot be used for online rectification. A technique is presented for online nonlinear filtering based on wavelet thresholding. OLMS rectification applies wavelet thresholding to data in a moving window of dyadic length to remove random errors. Gross errors are removed by combining wavelet thresholding with multiscale median filtering. Theoretical analysis shows that OLMS rectification using Haar wavelets subsumes mean filters of dyadic length, while rectification with smoother boundary corrected wavelets is analogous to adaptive exponential smoothing. If the rectified measurements are not needed online, the quality of rectification can be further improved by averaging the rectified signals in each window, overcoming the boundary effects encountered in TI rectification. Synthetic and industrial data show the benefits of the online multiscale and boundary corrected translation invariant rectification methods.
Chemo‐mechanical interaction in solid‐solid reactions
Authors: Bielenberg J. R. , Viljoen Hendrik J.
Published: April 2004

Deflagrations of solid reactants have been widely studied in the context of self‐propagating high‐temperature synthesis. Propagating velocities of the order of sound speed or faster have not been contemplated, due to the belief that the processes determining thermal conduction and diffusion cannot support those rates. Experiments, by Enikolopyan, Gogulya and others, however, disproved those notions. A model for solid‐solid reactions, which describe deflagration and detonation, is presented. The activation energy is large and the system requires significant preheating before ignition occurs. Based on experimental observations, the activation energy of a compressed system is lowered by the amount of elastic work done during compression. This phenomenon is included in the model. Ignition by impact and external thermal sources are investigated, and detonations develop only in the case of impact initiation. Interactions between the shock front and the reaction front are also investigated.
Advanced model for turbulent gas–solid flow and reaction in FCC riser reactors
Authors: Gao Jinsen , Xu Chunming , Lin Shixiong , Yang Guanghua , Guo Yincheng
Published: April 2004

A 3‐D two‐phase flow‐reaction model was developed for predicting the performance of FCC riser reactors. This model combines two‐phase turbulent flow with 13‐lump reaction kinetics. The simulated results show that the gas–particulate turbulent reacting flow regime in the riser reactor is extremely complex, especially at the feed inlet zone of the riser reactor. The distributions of velocities, temperature, and yields are not homogeneous in the riser reactor. The model predictions were used as a design tool for operational case studies. The optimum yields of desirable FCC products, gasoline and light fuel oil were achieved at a critical riser height that is less than 10 m above the feed inlet. Beyond the critical riser height, excessive cracking reactions resulted in the increased yields of by‐products, cracking gas and coke, at the expense of desirable products. Model simulations carried out to determine the parametric effects on product yields suggest that injection of water as a reaction‐terminating medium above the critical riser height can be an effective option for optimizing the yields of desirable products.
Laminar and turbulent shear‐induced flocculation of fractal aggregates
Authors: Flesch Jürgen C. , Spicer Patrick T. , Pratsinis Sotiris E.
Published: April 2004

A population balance model is used to model simultaneous coagulation and fragmentation in turbulent shear. The fractal‐like aggregate structure is quantified using a mass fractal dimension, Df = 2.05, derived from light‐scattering measurements and is incorporated into the model using appropriate kinetic expressions. In addition, fluid viscous effects on aggregate collisions are taken into account using existing theories on viscous retardation. Flocculation experiments of a polystyrene particle/Al(OH)3/water system in a stirred tank are compared to the model results. Good agreement is found for both average‐size evolution and steady‐state size distribution using only one fitting parameter and assuming binary breakage. The average polystyrene‐Al(OH)3 aggregate size initially increases before reaching a constant steady‐state value during coagulation–fragmentation in a stirred tank. Increasing the applied shear rate increases the coagulation and fragmentation rates, decreasing the steady‐state average aggregate size, and the time lag before steady state. The model developed in this work is applied to laminar shear data from the literature, showing excellent agreement.
Improvement of SRK equation of state for vapor‐liquid equilibria of petroleum fluids
Authors: Wang Li‐Sheng , Gmehling Jürgen
Published: April 2004

To improve the SRK equation of state for the calculation of vapor‐liquid equilibria of petroleum fluids, the parameters of the Mathias‐Copeman α‐function were refitted to a large number of experimental vapor‐pressure data (15,311 data points) stored in the Dortmund Data Bank with a particular view to improving the accuracy of the vapor‐pressure correlation in the low pressure range. Using the new parameters, vapor‐liquid equilibria were calculated using two different mixing rules: the group contribution mixing rule for the mixture parameters a of the SRK EOS; the conventional mixing rules for the mixture parameters a and b of the SRK EOS together with a characterization method developed on the basis of existing empirical correlations for the properties of plus fractions of undefined mixtures. The results show that the accuracy for crude oil and natural gas mixtures can be improved by refitting the SRK EOS parameters.
Generalized Doi–Ohta model for multiphase flow developed via generic
Authors: Wagner Norman J. , Öttinger Hans Christian , Edwards Brian J.
Published: April 2004

The model of Doi and Ohta for multiphase flow is developed and analyzed from a thermodynamic perspective using the general equation for the nonequilibrium reversible irreversible coupling (GENERIC) formalism. A procedure for obtaining a set of “thermodynamically consistent” transport equations is illustrated. The results demonstrate that the Doi–Ohta model is thermodynamically consistent and is thus a valid set of transport equations for multiphase flow. A modification of the original model is proposed to enable comparison with exact theories and validation with published simulation results, for which reasonable agreement is achieved. Extensions for more complex cases are derived within the GENERIC framework. The derivation demonstrates the ability of the GENERIC formalism to improve the theoretical basis of models for transport in complex fluids.
Fast numerical simulation for full bore rupture of pressurized pipelines
Authors: Mahgerefteh Haroun , Saha Pratik , Economou Ioannis G.
Published: April 2004

An efficient numerical simulation (CNGS‐MOC), based on the method of characteristics for simulating full bore rupture of long pipelines containing two‐phase hydrocarbons, was developed. The use of curved characteristics, in conjunction with a compound nested grid system, as well as a fast mathematical algorithm, lead to a significant reduction of CPU time, while improving accuracy. The model is validated extensively against field data including those obtained during the Piper Alpha tragedy, as well as the Isle of Grain depressurization tests. Its predictions are compared with those based on other mathematical models including PLAC, META‐HEM, MSM‐CS, as well as BLOWDOWN. Both CNGS‐MOC and META‐HEM produce reasonably accurate predictions with the remaining models assessed performing relatively poorly.
Effects and control of humidity and particle mixing in fluid‐bed granulation
Authors: Schaafsma Stefan H. , Kossen Nico W. F. , Mos Marinus T. , Blauw Levy , Hoffmann Alex C.
Published: April 2004

The novel technique of spraying binder liquid in pulses of short duration on a bubbling fluidized bed was used to study the effect liquid distribution, mixing, and relative humidity has on granule growth. Two important mixing zones in the fluid‐bed granulation process are identified. First, the wetting zone, where the spraying rate relative to the surface renewal rate determines the wetting of the granules and powder. The surface renewal has to be sufficient to prevent overwetting of the surface, which would result in defluidization of the bed. Second, the bulk zone, where the turnover rate of granules relative to the drying rate may determine the secondary growth rate of granules. The granule growth is influenced by the adsorption of moisture on the fluidized particles governed by the relative humidity of the interstitial gas, due to enhancement of interparticle forces and reduces the mixing intensity. Spraying with pulses has a great potential for controlling the granulation process.
Molecular‐weight distribution attainable in a batch free‐radical polymerization
Authors: Yoo Kee‐Youn , Rhee Hyun‐Ku
Published: April 2004

The attainable values of molecular‐weight distribution (MWD) specified by the degree of polymerization and the polydispersity in a batch free‐radical polymerization reactor are described. The two‐step method based on the quadratic profile of the instantaneous degree of polymerization is used to obtain the recipe producing the polymer with the desired MWD specification. The analytic solution for the quadratic profile is derived and analyzed in order to estimate the attainable MWD given the constraints on the range of the instantaneous degree of polymerization. The entire computational procedure is intelligently organized, and thus the actual computational load is very low compared to the conventional optimization scheme. The MWD attainable under a specific operating condition with constraints in a batch styrene polymerization reactor is visualized and various reactor temperature trajectories that would produce the desired polymer product are calculated. The ability to predict the attainable MWD allows an engineer to easily design the recipes that would meet the desired MWD specification.
Optimal design of a subsurface redox barrier
Authors: Chilakapati Ashok
Published: April 2004

Harmful contaminants such as chromium (Cr+6), and TCE can be removed from groundwater by reactions with reduced subsurface sediments. Establishing an in situ Fe(II) barrier through the reduction of soil‐bound Fe(III) to Fe(II) by injecting a sodium dithionite (Na2S2O4) solution is studied. Critical to this problem is the possible formation and expansion of a zone around the injection, where all the soil‐bound Fe(III) is reduced to Fe(II). Different reaction models apply inside and outside of this zone so that a determination of this moving boundary is a fundamental part of the solution. The complete analytic solution to this problem was used to develop optimal process parameters, such as injection rate and operational time, that maximize the radius of the Fe(III)‐reduced zone when a given mass of sodium dithionite is injected at a well. When a large reduction [>63% of initially present Fe(III)] is desired, the results indicate that it is better to use a low flow rate to form a Fe(III)‐free zone around the injection. The opposite is true for smaller reductions (<63%), so that a faster injection rate that avoids the formation of the Fe(III)‐free zone yields a larger reduction zone.
Dynamics of two‐phase flow in porous media: Simultaneous invasion of two fluids
Authors: Hashemi Mehrdad , Dabir Bahram , Sahimi Muhammad
Published: April 2004

Models of two‐phase flow and displacement in porous media developed so far typically involve one displacing (invader) and one displaced (defender) fluid. However, in many important applications of these phenomena at field scales, such as two‐phase flow in fractured porous media, as well as in laboratory studies, require at least two invaders which also act as the defenders. The results of extensive Monte Carlo simulations of a novel model of such phenomena are reported. The porous medium is represented as a network of pore throats and pore bodies to which effective sizes are assigned that are selected from a given distribution. Both 2‐D and 3‐D networks are used. The simulation results indicate that the structure of the fluids' clusters is volatile, that is, it changes with the time t and length scale. Moreover, ns (s, t), the number of fluid clusters of size s, 〈s(t)〉, the mean cluster size, and S(t), the saturation of the fluids all vary with t in a manner that resembles an oscillatory behavior. This behavior is caused by the dynamic breakup and recoalescence of the fluids' clusters, which is a result of simultaneous invasion of the two fluids. The flow effect of thin wetting fluid films on the dynamics of the displacement is strong over a broad range of the capillary number. Novel dynamical scaling laws for the cluster‐size distribution are obtained. Some results agree qualitatively with the experimental observations, while others provide rational explanations for some unexplained data.
Effect of fluid motion on the aggregation of small particles subject to interaction forces
Authors: Melis Stefano , Verduyn Marcel , Storti Giuseppe , Morbidelli Massimo , Bałdyga Jerzy
Published: April 2004

The effect of bulk fluid motion on aggregation of small particles subject to interaction forces of the DVLO type is investigated. In particular, the convection‐diffusion equation for the probability of finding two particles at a given relative position is solved numerically for the case of an axisymmetrical linear flow field, which approximates a turbulent flow field. The effect of bulk motion is shown to be important particularly in the case of thick double layers, where a critical size exists above which aggregation exhibits a self‐accelerating behavior, that is, the aggregation rate constant increases with particle size. On the other hand, systems with thin double layers appear to be almost unaffected by shear. An approximate criterion for distinguishing between these two cases is presented.
Necessary condition for pulsing flow inception in a trickle bed
Authors: Cheng Zhen‐Min , Yuan Wei‐Kang
Published: April 2004

Based on the three existing forms of liquid in a trickle bed, as proposed by Charpentier et al. (1968a,b), a straight channel model is developed with a wall constituted from the packing. Within the channel, liquid and gas phases are assumed to flow concurrently downward at constant velocities, just as in an empty pipe. Since the channel wall is assumed to be formed from the packing, the capillary influence could be taken into account. Capillary and frictional forces are considered as the two principal factors which determine the stability of the liquid film covering a pellet. Evaluating the balance of these two factors gives rise to a new model, which is considered the necessary condition for pulsing flow inception corresponding to the original work by Ng (1986).
Particle mixing in rotating fluidized beds: Inferences about the fluidized state
Authors: Qian Gui‐Hua , B´gyi Istv´n , Pfeffer Robert , Shaw Henry , Stevens John G.
Published: April 2004

Particle motion in a rotating fluidized bed was studied by observing the mixing of two layers of particles of different colors. The particles in the two layers were either nearly identical, except for their color, or were of different density and size distribution. All of the particles were in the Geldart‐A classification for conventional fluidized beds. After fluidization, but before mixing, the bed exhibited some fluid‐like behavior, that is, the inner surface became radially uniform. For particle layers of the same material, mixing occurred after Umfc was reached, with bubbles observed at the mixing velocity. No experimental difference between Umb and Umfc could be discerned; they either coincided or were nearly equal. Bubbles appeared to be responsible for particle motion and mixing. When the denser particles are placed on the distributor, the mixing behavior was similar to that observed for layers of the same material. However, when the less dense particles are placed on the distributor, mixing is dominated by differences in density and occurs before bubbles are visible.
Simulated moving‐bed units with extra‐column dead volume
Authors: Migliorini Cristiano , Mazzotti Marco , Morbidelli Massimo
Published: April 2004

Simulated moving‐bed (SMB) technology is attracting increased interest for application to the separation of pharmaceuticals and fine chemicals, particularly to enantioseparations. Unlike the large‐scale UOP Sorbex process, these small‐scale units use a series of HPLC columns, and the volume of the connecting lines may become comparable to the volume of the columns. In this work the effect of extra‐column dead volume on SMB performance is analyzed, with the aim of providing a conceptual framework to account for it. In particular, guidelines to design SMB operating conditions with nonnegligible extra‐column dead volumes to achieve the desired separation performance are provided. These are quantitatively assessed through comparison with the results obtained using a detailed SMB model, which accounts for extra‐column dead volumes. The model allows the extent and the role of extra‐column band broadening due to the presence of dead volumes to be evaluated. A set of experimental results on a laboratory SMB unit is discussed in light of the theoretical findings.
Nonisothermal model for gas separation hollow‐fiber membranes
Authors: Coker D. T. , Allen T. , Freeman B. D. , Fleming G. K.
Published: April 2004

A model for multicomponent gas separation using hollow‐fiber membrane modules is presented that explicitly accounts for heating or cooling inside membrane permeators due to gas expansion. The model permits simulation of countercurrent contacting with permeate purging (or sweep). The numerical approach permits rapid and stable solutions for cases with many components, even when the mixture contains components with widely varying permeability coefficients. Simulation results are presented for natural‐gas sweetening, a commercially significant application, using polymer permeation properties similar to those of a high‐performance polyimide. For some conditions, temperature decreases from the feed to residue end of the module by as much as 40°C. As CO2 concentration in the feed increases or as stage cut increases, the temperature decrease from feed to residue increases. Relative to an isothermal case, expansion‐driven cooling reduces stage cut at a given feed flow rate since gas permeability decreases with decreasing temperature. Neglect of expansion‐driven cooling in natural‐gas separation simulations can lead to large errors in estimating the amount of feed gas that can be treated to achieve a fixed residue composition. For 30% CO2 feed concentration, if the effective membrane thickness is halved, only a 20% increase (rather than almost a factor of 2) in the amount of gas that can be treated per unit area is obtained due to the impact of expansion‐driven cooling on gas flux.
Optimal design of crystallization‐based separation schemes
Authors: Cisternas Luis A.
Published: April 2004

A network flow model is presented for synthesizing crystallization‐based separations for multicomponent systems. The construction of the network flow is based on the identification of feasible thermodynamic states. The method allows consideration of several operation temperatures, complex solid–liquid equilibrium behavior, and several multicomponent feeds and products. For systems with two solutes a linear programming model is obtained, while for systems with three or more solutes a nonlinear programming model is obtained. The relative composition diagram is proposed to determine feasible operation points. For a small problem the relative composition diagram can be used to recognize flowsheet alternatives. For large problems, the relative composition diagram allows one to decrease the nodes in the network, thus decreasing the execution time. The technique is illustrated with three example problems.
Performance and stability analysis of LP‐MPC and QP‐MPC cascade control systems
Authors: Ying Chao‐Ming , Joseph Babu
Published: April 2004

Model predictive control (MPC) is used extensively in industry to optimally control constrained, multivariable processes. For nonsquare systems (with more inputs than outputs), extra degrees of freedom can be used to dynamically drive the process to its economic optimum operating conditions. This is accomplished by cascading a local linear programming (LP) or quadratic programming (QP) controller using steady‐state models. Such a cascade control scheme (LP‐MPC or QP‐MPC) continuously computes and updates the set points used by the lower‐level MPC algorithm. While this methodology has been in use by industry for many years, its properties have not been addressed in the literature. The properties of such cascaded MPC systems are analyzed from the point of view of implementation strategies, stability properties, and economic and dynamic performance. Some theoretical results on stability are derived along with a case study involving the Shell control problem.
Smoothing methods for complementarity problems in process engineering
Authors: Gopal Vipin , Biegler Lorenz T.
Published: April 2004

Complementarity is central to all constrained optimization problems. However, direct enforcement of complementarity conditions is difficult because of the inherent nondifferentiability associated with them. Here, a class of smoothing methods for solving the complementarity problem by using a continuation algorithm to solve nonlinear equations is studied. The applicability of smoothing methods to approximate complicated nested derivative discontinuities is investigated using simple functions with a single smoothing parameter. In addition, an equation‐based formulation for solving the phase equilibrium problem with complementarity conditions is formulated. This approach can model the appearance and disappearance of phases directly in phase equilibrium problems. Moreover, it is shown how smoothing methods can be used to solve limiting distillation cases, such as dry and vaporless trays, modeled within an equation‐based formulation.
Kinetics on a supported catalyst at supercritical, nondeactivating conditions
Authors: Clark Michael C. , Subramaniam Bala
Published: April 2004

Intrinsic kinetic parameters are reported for the Pt/γ‐Al2O3 catalyzed geometric isomerization of 1‐hexene (Pc = 29.7 bar; Tc = 231°C) under conditions of steady catalyst activity. Deaerating the hexene feed, pretreating it in a bed of activated alumina to reduce the peroxide impurities to 2 ppm or less, and passivating the reactor surface (by silicosteel coating) reduce the formation of hexene oligomers (coke precursors) by about an order of magnitude. This, in conjunction with operation at supercritical conditions (235–304°C, 35–70 bar), results in steady isomerization activity with no measurable coke laydown. Arrhenius plots of rate constants, estimated from the steady isomerization rates, yield an intrinsic activation energy of roughly 109 kJ/mol. This value is up to twofold greater than those reported for similar reaction systems, but on a deactivating catalysts. Our results confirm that optimum conditions that maximize catalyst activity and effectiveness factor exist in the near‐critical region with catalyst activity and effectiveness factor.
Use of hydration and dissociation chemistries with the electrolyte–NRTL model
Authors: Chen Chau‐Chyun , Mathias Paul M. , Orbey Hasan
Published: April 2004

Sound theoretical understanding and accurate mathematical representation of electrolyte solution nonideality were the subject of extensive research efforts for decades. It is demonstrated that these goals can be achieved by incorporation of hydration and partial dissociation chemistries with the semi‐empirical electrolyte NRTL model. The resulting model provides a comprehensive thermodynamic framework for accurate correlation and reliable extrapolation of electrolyte solution nonideality over the entire concentration range.
Shortcut methods for nonideal multicomponent distillation: 2. Complex columns
Authors: von Watzdorf Rüdiger , Bausa Jürgen , Marquardt Wolfgang
Published: April 2004

In an earlier article of the authors, the rectification body method was introduced for rapidly calculating the minimum reflux of nonideal and azeotropic multicomponent separations. Here, the RBM is generalized to two important classes of complex columns: (1) columns having sidestreams, and (2) columns with one or several side‐enricher and/or side‐stripper columns. As in the case of simple columns, the method is entirely general and does not require any a priori information on the limiting pinches prevailing in the various column sections.
Synthesis of bulk solids processing systems
Authors: Wibowo Christianto , Ng Ka M.
Published: April 2004

A systematic procedure for the synthesis of bulk solids processing systems guides the user in an evolutionary manner to generate flowsheet alternatives to transform a solids feed with given attributes into products with desired attributes. The first step is to select a functional structure based on the primary objective of the processing system. This is followed by the selection of equipment for the functional structure. In the third step, the functional structure is expanded by adding other necessary unit operations to form a flowsheet. This is accompanied by the assignment of a suitable equipment unit to each unit operation. Storage and transportation units are then added to the flowsheet in the fourth step. Rules and heuristics to help make decisions at each step are presented, as well as simulations used to evaluate flowsheet alternatives. Design variables are identified and short‐cut models based on discretized population balance equations are used to quantify equipment performance. The procedure is illustrated with commercial examples including the production of potash, pesticide granules, aspirin and an azo dye.
Feasibility issues in linear model predictive control
Authors: Scokaert Pierre O. M. , Rawlings James B.
Published: April 2004

The problem of infeasibility caused by inconsistent hard state constraints in model predictive control is considered. The difficulties that may arise are analyzed, and the inherent trade‐off a controller must address at times of infeasibility is shown. From a multiobjective viewpoint, it is demonstrated that the two approaches for handling infeasibility documented in the literature have significant limitations. Two new techniques for handling infeasibility that overcome the difficulties encountered with current approaches are presented.
Improved PCA methods for process disturbance and failure identification
Authors: Wachs Amir , Lewin Daniel R.
Published: April 2004

Principal component analysis (PCA) is a powerful technique for constructing reduced‐order models based on process measurements, obtained by the rotation of the measurement space. These models can be subsequently utilized for chemical‐process monitoring, particularly for disturbance and failure diagnosis. Since the standard PCA procedure does not account for the time‐dependent relationships among the process variables, this leads to poorer disturbance isolation capability in dynamic applications. A simple idea, in which the last s PCA scores are recursively summed and used to construct descriptive statistics for process monitoring, is presented. Analytically, it is shown that the disturbance resolution afforded is enhanced as a result. Resolution is improved further through the use of an algorithm that enhances the correlations between the input and output variables through optimal time shifting. An overall strategy for on‐line monitoring developed includes disturbance identification through mapping. The approach is demonstrated by two industrially relevant case studies.
Targeting procedures for energy savings by heat integration across plants
Authors: Rodera Hernán , Bagajewicz Miguel J.
Published: April 2004

Heat integration across plants can be accomplished either directly using process streams or indirectly using intermediate fluids. By applying pinch analysis to a system of two plants, it is first shown that the heat transfer leading effectively to energy savings occurs at temperature levels between the pinch points of both plants. In some cases, however, heat transfer in other regions is required to attain maximum savings. A systematic procedure to identify energy‐saving targets is discussed, as well as a strategy to determine the minimum number of intermediate‐fluid circuits needed to achieve maximum savings. An MILP problem is proposed to determine the optimum location of the intermediate fluid circuits. The use of steam as intermediate fluid and extensions to a system of more than two plants are discussed.
Chance‐constrained model predictive control
Authors: Schwarm Alexander T. , Nikolaou Michael
Published: April 2004

This work focuses on robustness of model‐predictive control with respect to satisfaction of process output constraints. A method of improving such robustness is presented. The method relies on formulating output constraints as chance constraints using the uncertainty description of the process model. The resulting on‐line optimization problem is convex. The proposed approach is illustrated through a simulation case study on a high‐purity distillation column. Suggestions for further improvements are made.
Synthesis and implementation of procedural controllers for event‐driven operations
Authors: Sanchez A. , Rotstein G. , Alsop N. , Macchietto S.
Published: April 2004

A framework is presented for synthesizing logic feedback controllers for event‐driven operations, which are used typically for startup and shutdown operations, emergency procedures, and alarm handling. The framework encompasses techniques for discrete‐event modeling of open‐loop process behavior and operational specifications, as well as the synthesis of feedback control mechanisms called procedural controllers. A procedural controller, if it exists, is mathematically guaranteed to satisfy its operational specifications. This is of particular importance for control systems in which high integrity and correctness are required by design (such as systems in which human life is at risk). The notions introduced and the framework presented are illustrated with a small example. The applicability of the framework to cases of industrial complexity is demonstrated by synthesizing a procedural controller and implementing it as a control code for a section of the operation of an automated multipurpose‐multiproduct batch pilot plant.
Use of Buckingham potentials in engineering approximations for chemical kinetics
Authors: Blowers Paul , Masel Richard I.
Published: April 2004

Previous investigators have often used the Polanyi relationship to correlate activation barriers for series of reactions. The correlations often work, but a different Polanyi relationship is needed for each class of reactions. As a result, the correlations are difficult to extrapolate to new situations. In this article, we suggest new correlations that show greater capability to correlate rate data. Quantum mechanical calculations are done which indicate that the main barriers to ligand exchange reactions are associated with getting the reactants close enough to react. The energy to get the reactants close enough to react can be calculated from a united‐atom Buckingham potential and correlated to rate data. The additional correlating parameter allows the activation energies for a series of hydrocarbon reactions to be collapsed onto a single line. The root‐mean‐square error of the line is 2 kcal/mol compared to 7 kcal/mol with the Polanyi relationship alone. This shows that our new correlating parameter will be useful in engineering approximations in kinetics.
Kinetics of the aqueous‐phase reaction between ozone and 2,4,6‐trichlorophenol
Authors: Rinker Edward B. , Ashour Sami S. , Johnson Michael C. , Kott Gregory J. , Rinker Robert G. , Sandall Orville C.
Published: April 2004

A wetted‐sphere absorber was used to measure absorption rates of ozone into aqueous‐buffered solutions of 2,4,6‐trichlorophenol (TCP). Gas consisting of approximately 2.5% ozone in oxygen was contacted with aqueous‐buffered TCP solution flowing over a sphere in a laminar liquid film. Absorption data were obtained by measuring the change in the liquid‐phase concentration of TCP from the inlet to the outlet of the absorber. A rigorous numerical model for this diffusion/reaction process was used to analyze the absorption data to determine the second‐order rate constant of the reaction between ozone and TCP at 15–35°C and pH 2 and 7. The kinetic data indicate that the rate‐limiting step for the reaction of aqueous TCP with ozone is the same at pH 2 and 7. Four reaction products that occur early in the reaction of ozone with aqueous TCP were identified by GCMS, including: 2,3,4,6‐tetrachlorophenol (TRCP), 4,6‐dichlorocatechol (DCC), 2,6‐dichlorohydroquinone (DCHQ), and 2,6‐dichloroquinone (DCQ). Only DCHQ and DCQ occurred in significant concentrations. Over the pH range of 2 to 5, both DCHQ and DCQ were detected in the reacted solutions, while for pH 6 and 7 only DCHQ was detected. Carbon‐13 NMR analysis shows that over longer ozone exposure times short‐chain carboxylic acids are the dominant species in the reacted TCP solutions.
Synthesis of prepolymerization stage in polycondensation processes
Authors: Samant Ketan D. , Ng Ka M.
Published: April 2004

A systematic design procedure of the prepolymerization stage of a polycondensation process is presented. This procedure treats polycondensation processes as reactive separation processes and offers an alternative to design through simulations. Feasible flowsheets and operating conditions are developed by analyzing the phase behavior of polycondensation systems at thermodynamic equilibrium. Functional end groups and apparent mole fractions are used to represent reaction kinetics and equilibrium, and real mole fractions are used to represent phase equilibrium. Reactive phase diagrams are plotted in transformed mole‐fraction space to facilitate visualization of the phase behavior. Three examples are presented: manufacture of polyethylene terephthalate (PET) from dimethyl terephthalate (DMT) and ethylene glycol (EG), from terephthalic acid (TPA) and EG, and manufacture of nylon‐6,6. For these processes, the results of the approach agree excellently with detailed iterative performance simulations. The approach also reveals additional feasible process alternatives and corresponding operating conditions.
Flow and dispersion in porous media: Lattice‐Boltzmann and NMR studies
Authors: Manz B. , Gladden L. F. , Warren P. B.
Published: April 2004

The results of lattice‐Boltzmann simulations of the flow field through a random packing of spheres are compared with NMR velocimetry and propagator measurements. Flows are investigated for Péclet (Pe) and Reynolds (Re) numbers in the range 182<Pe<350 and 0.4<Re<0.77, respectively. An MRI visualization of the 3‐D packing of spheres for which NMR data are obtained is used as the matrix for the simulation, thereby enabling a direct assessment of the ability of the lattice‐Boltzmann method to quantitatively predict flow phenomena within porous media of complex geometry. By introducing normalized parameters, hydrodynamical dispersion over normalized displacement lengthscales extending over nearly four orders of magnitude is investigated and is shown to be dominated by mechanical dispersion over most of this range. At the largest lengthscales studied holdup may play a significant role in the hydrodynamics characteristic of the porous medium. Quantitative agreement between NMR measurements and the predictions of the lattice‐Boltzmann simulation is obtained in all cases.
New effective‐nuclei concept for simplified analysis of batch crystallization
Authors: Yokota Masaaki , Sato Akira , Kubota Noriaki
Published: April 2004

A new “effective nuclei” concept for a simplified approach to the analysis of batch crystallization with agglomeration is proposed. The new effective‐nuclei concept comes from agglomeration characteristics observed experimentally in a batch crystallizer. Small‐sized crystals could combine with both large‐sized crystals and each other. Agglomeration among large‐sized crystals can be neglected. From these experimental results, new effective nuclei are defined as crystals at a size‐boundary between small‐sized crystals and large‐4sized crystals. By taking into account only large‐sized crystals in formulating population balance equation, the agglomeration term can be omitted. The analytical solution of the population balance equation is induced for an ideal batch‐cooling case (constant supersaturation and constant effective growth rate of crystal). The solution explains measured data well.
Adsorption kinetics of light paraffins in AC by a constant molar flow‐rate method
Authors: Prasetyo I. , Do D. D.
Published: April 2004

A semibatch constant molar flow‐rate technique developed earlier by Do was applied to a series of light paraffins adsorbing onto a number of adsorbents. The adsorption and diffusion of methane, ethane, propane and n‐butane on activated carbon at different temperatures were studied to determine the diffusivity of surface diffusion and its activation energy. It was found that the surface diffusivity decreases with the molecular weight in an exponential manner and increases with temperature according to the Arrhenius law. The ratio of the activation energy for surface diffusion to the heat of adsorption increases with the carbon number. A mechanism for surface diffusion is suggested to explain this increase.