bluuebunny/crossref_metadata_2025_split huggingface dataset:
Complex dynamic features of a cooled reverse‐flow reactor
Authors: Khinast J. , Gurumoorthy A. , Luss D. 
Published: April 2004

AbstractA reverse‐flow reactor (RFR) usually attains a symmetric period‐1 state, so that the temperature profile just after a flow reversal is a mirror image of that after the previous flow reversal. Recent simulations show that in certain cases a cooled RFR may attain other states with different types of periodicity or even complex quasi‐periodic states. The maximum temperature of these states often exceeds that of the symmetric states so that it may deactivate the catalyst and/or lead to safety problems. A systematic, numerically efficient method is presented for constructing maps of parameter regions in which a cooled RFR has qualitatively different dynamic features. The technique is applied to determine the dependence of these dynamic features on the cooling capacity and flow‐reversal period. Stable quasi‐periodic and asymmetric period‐1 states exist mainly for short flow‐reversal periods. The quasi‐periodic states usually exist for lower cooling capacities than those for which the asymmetric period‐1 states exist. Stable symmetric and asymmetric period‐1 states exist for the same set of parameters in very narrow regions of the parameter space. The behavior of the RFR in the limit of very fast flow reversals is usually modeled by a countercurrent flow reactor, with equal flow rates in the two compartments. This cooled reactor may attain asymmetric states for certain sets of parameters.
Pseudo‐adiabatic operation and runaway in tubular reactors
Authors: Wu Hua , Morbidelli Massimo , Varma Arvind 
Published: April 2004

AbstractCritical conditions for thermal runaway in homogeneous tubular reactors with either constant or cocurrent external cooling were investigated through the applications of the generalized sensitivity criterion using reactor axial coordinate as the independent variable, and compared with those predicted by geometry‐based criteria. All criteria are found to provide rather similar results when a hot spot is present inside the reactor. When the hot spot shifts to the reactor outlet, leading to the so‐called pseudo‐adiabatic operation (PAO), the geometry‐based criteria become either invalid or conservative with respect to the generalized one. In addition, when the generalized criterion is applied to the cases where the PA0 is present, it is required to use the axial coordinate rather than reactant conversion as the independent variable to predict the critical conditions, and the obtained runaway boundaries in the reactor parameter plane coincide with the PA0 boundaries.
Molecular‐thermodynamic framework for asphaltene‐oil equilibria
Authors: Wu Jianzhong , Prausnitz John M. , Firoozabadi Abbas 
Published: April 2004

AbstractAsphaltene precipitation is a perennial problem in producing and refining crude oils. To avoid precipitation, it is useful to know the solubility of asphaltenes in petroleum liquids as a function of temperature, pressure and liquid‐phase composition. In the novel molecular‐thermodynamic framework presented here, both asphaltenes and resins are represented by pseudo‐pure components while all other components in the solution are represented by a continuous medium that affects interactions among asphaltene and resin particles. The effect of the medium on asphaltene‐asphaltent: resin‐asphaltene, resin‐resin pair interactions is taken into account through its density and dispersion‐force properties To obtain expressions for the chemical potential of asphaltene and for the osmotic pressure of an asphaltene‐containing solution, the SAFT model is used in the framework of McMillan‐Mayer theory, which considers hard‐sphere repulsive, association and dispersion‐force interactions. By assuming that asphaltene precipitation is a liquid‐liquid equilibrium process, a variety of experimental observations can be explained, including effects of temperature, pressure, and composition on the phase behavior of asphaltene‐containing fluids. For practical quantitative applications, the model outlined here requires molecular parameters that must be estimated from a few experimental data.
Simulation of flow and mixing of particles in a rotating and rocking cylinder
Authors: Wightman Carolyn , Moakher Maher , Muzzio Fernando J. , Walton O. 
Published: April 2004

AbstractGranular material in a cylindrical vessel undergoing rotational and rocking motions is modeled using a discrete element method. Rotational motion supplemented by rocking is compared to purely rotational motion via linear density profiles, velocity fields, and axial concentration profiles. The rocking motion, which imparts a time‐dependent flow perturbation to purely rotational motion, dramatically enhanced mixing in laboratory studies. Simulation results appear to agree well with experimental results and observations.
Synthesis of extractive reaction processes
Authors: Samant Ketan D. , Ng Ka M. 
Published: April 2004

AbstractA systematic method presented synthesizes extractive reaction processes. The objective is to develop liquid‐phase processes involving simultaneous reaction and separation by extraction to achieve improved yield, selectivity to a desired product, and separation of byproducts. The method is based on the equilibrium thermodynamic analysis of multicomponent reactive systems with multiple liquid phases. Using transformed coordinates, systems with three or fewer degrees of freedom can be conveniently analyzed regardless of the number of components in the system. Key features of the reactive phase diagrams relevant to achieving the process objectives are identified. The method is illustrated with several model examples to demonstrate the advantages of extractive reaction over conventional single‐phase processes.
Microhydrodynamics of sharp corners and edges: Traction singularities
Authors: Mustakis I. , Kim S. 
Published: April 2004

AbstractSharp corners and edges are ubiquitous in modem technology and found on the tip of an atomic force microscope (AFM) and the vertices and edges of clystalline materials. The microhydrodynamics of particles with sharp corners and edges are special in that geometric discontinuities give rise to traction singularities for viscous flow around the particles. Such traction fields are analyzed with both analytical and numerical solution of a second kind integral equation. The traction field in the vicinity of a vertex has a singular structure that is more intricate than simple superposition of the well‐known edge singularities. The new insights into the structure of the vertex singularities are used to formulate and test spectral element strategies for numerical solution of viscous flow past sharp corners and edges. The best schemes constructed to date show remarkable agreement with the analytical results.
Growth regime map for liquid‐bound granules
Authors: Iveson S. M. , Litster J. D. 
Published: April 2004

AbstractA regime map of granule growth behavior is proposed based on granule deformation during collision and the granule liquid content measured as the maximum pore saturation. The granule deformability on collision is represented by a deformation number, which is a ratio of granule impact energy to the plastic energy absorbed per unit strain. Granule growth regimes such as steady growth, induction, nucleation, crumb, and slurry are defined. This regime map qualitatively explains the uariatiom in granulation behauior. Laboratoly drum granulation experiments were used to test the regime map. Experiments were peqformed in a 0.3‐m‐dia. drum using three sizes of glass ballotini (19, 31, and 60 μm) with water and glycerol as liquid binders. Increasing granule yield stress by decreasing particle size and increasing binder viscosity caused the system to move from steady growth to induction behauior as predicted by the regime map. Preliminary validation with literature data was also encouraging. More work, howeuer, is required to better quantify the boundaries between different growth regimes and to investigate the effect of process agitation intensiq. This regime map has great potential to help design and control granulation systems, because it is based on properties of the powder/binder qstem that can be measured or estimated without peqforming any granulation tests.
Influence of Nernst‐Planck diffusion on hollow‐fiber mass‐transfer processes
Authors: Qin Yingjie , Cabral Joaquim M. S. 
Published: April 2004

AbstractHollow‐fiber membrane reactive extraction and facilitated supported liquid membrane processes for the separation of charged species are analyzed theoretically. A mathematical model incorporating laminar flow, Nemst‐Planck diffusion, buffer effect, ion strength, interfacial reaction kinetics and/or equilibria, diffusivity of species in the membrane phase, shell resistances, and shell concentration is deueloped and solued numerically. When the lumen mass‐transfer resistance is dominant, the predicted remoual rate of a single ion or the separation rate of a mixture of ions by a simplified model with Fickian diffusion deuiates greatly from the prediction with the present model. For instance, when the concentration of triualent ions in the feed decreases to half of its initial ualue, the membrane length calculated using Fickian diffusion is 75% higher than the length predicted by Nemst‐Planck diffusion. Thus, the Leueque equation based on Fick's law does not adequately describe the mass transfer of the charged species in the lumen feed. The effect of buffer ions and diffusiuity of species in the membrane phase on the mass transfer of the species is also discussed.
Adsorptive separations using supercritical frontal analysis chromatography
Authors: Cross William M. , Akgerman Aydin 
Published: April 2004

AbstractMany food products and pharmuceuticals industries are looking to integrating super‐critical extraction and separations processes to fulfill processing needs due to limitations and regulations concerning the use of organic solvents. One such technology, which may affect these industries, is continuous supercritical adsorptive separation, a processing technology similar to the separation process of xylenes developed at UOP. Many products like pharmaceuticals and other human consumables could be processed more economically and with higher purity by supercritical fluid adsorption/desorption processes. Past studies, however, have focused on single‐component adsorption/desorption involving a supercritical mobile phase, whereas adsorptive separations involving super‐critical fluids are multicomponent systems. Accurate experimental techniques, which can determine both multicomponent solubilities in supercritical fluids and multicomponent adsorption isotherms in the presence of supercritical fluids, were developed to investigate the multicomponent supercritical adsorption phenomenon. A dynamic column model developed takes into account column dispersion as well as mass transfer and diffusive resistances. Experimental isotherm data incorporated into the model can predict the breakthrough profiles. An extension of pulse chromatography was used to identify the important hydrodynamic and transport parameters, and operating conditions are identified for optimal separations.
Hollow‐fiber‐based adsorbers for gas separation by pressure‐swing adsorption
Authors: Feng Xianshe , Pan Chuen Y. , McMinis Curtis W. , Ivory John , Ghosh Dave 
Published: April 2004

AbstractHollow‐fiber‐based adsorbers for gas separation by pressure‐swing adsorption (PSA) was studied experimentally. The high efficiency of hollow‐fiber‐based adsorbers for gas separation was illustrated by hydrogen separation using fine‐powder‐activated carbon and molecular sieve as adsorbents. The adsorption equilibrium and dynamics of the hollow‐fiber adsorbers were determined. The pressure drop of the gas flowing through the adsorbers was also examined. The adsorbers were tested for hydrogen separation from nitrogen, carbon dioxide, and a multicomponent gas mixture simulating ammonia syn‐ thesis purge gas. The PSA systems using the hollow‐fiber adsorbers were very effective for hydrogen purification. The high separation efficiency is derived from the fast mass‐ transfer rate and low pressure drop, two key features of hollow‐fiber‐based adsorbers.
Nonlinear model reduction for control of distributed systems: A computer‐assisted study
Authors: Shvartsman Stanislav Y. , Kevrekidis Ioannis G. 
Published: April 2004

AbstractModel reduction methodologies for the partial differential equations modeling distributed parameter systems constitute an important first step in controller design. A systematic computer‐assisted study illustrating the use of two such methodologies (Approximate Inertial Manifolds and the Karhunen‐Loève expansion) in controlling (stabilizing) a nonlinear reaction‐diffusion problem is presented. The approximation quality of the models, issues of computational implementation of the reduction procedure, as well as issues of closed‐loop stability are addressed.
Cyclic scheduling of continuous parallel‐process units with decaying performance
Authors: And Vipul Jain , Grossmann Ignacio E. 
Published: April 2004

AbstractThe problem of scheduling multiple feeds on parallel units is addressed. The performance of each unit is assumed to decrease with time, therefore requiring planned maintenance to restore its performance. A mixed‐integer nonlinear programming (MINLP) model presented uses the process information regarding the exponential decay in performance with time to find a cyclic schedule for feed processing. Optimization trade‐off is between the performance of the units, and the maintenance costs and the loss in production due to shutdowns. Three main decisions for the optimal schedule are: a) assignment of various feeds to different units; b) processing time of each feed on different units; and c) number of subcycles of each feed on diferent units. Special mathematical properties of the MINLP model are exploited to solve the problem to global optimality with a branch‐and‐bound method. Using several examples advantages of the model are illustrated and performances of different solution algorithms are compared.
Optimal synthesis of stagewise continuous crystallization process networks
Authors: Sheikh Ahmad Y. , Jones Alan G. 
Published: April 2004

AbstractA new synthesis method based on targeting is presented for the design of stagewise continuous crystallization processes. The methodology determines the optimal flow distribution to each of the crystallizers in addition to other common control variables, including temperature, concentration, and vessel volume. The resulting optimization model yields a boundary‐ualue differential equation system, which is transformed into a set of nonlinear equations through orthogonal collocation and solved numerically. For the synthesis of the continuous KNO3, crystallization process considered here, the predicted optimal network solution shows that the common tradeoff between high yield and large average crystal sizes has been successfilly eradicated by the new design approach.
Effects of imperfect mixing on low‐density polyethylene reactor dynamics
Authors: Villa Carlos M. , Dihora Jiten O. , Harmon Ray W. 
Published: April 2004

AbstractEarlier work considered the effect of feed conditions and controller configuration on the runaway behavior of LDPE autoclave reactors assuming a perfectly mixed reactor. This study provides additional insight on the dynamics of such reacton by using an imperfectly mixed reactor model and bifurcation analysis to show the changes in the stability region when there is imperfect macroscale mixing. The presence of imperfect mixing substantially increases the range of stable operation of the reactor and makes the process much easier to control than for a perfectly mixed reactor. The results of model analysis and simulations are used to identify some of the conditions that lead to unstable reactor behavior and to suggest ways to avoid reactor runaway or reactor extinction during grade transitions and other process operation disturbances.
Model for the sulfidation of calcined limestone and its use in reactor models
Authors: Heesink A. B. M. (Bert) , Brilman D. W. F. (Wim) , van Swaaij W. P. M. (Wim) 
Published: April 2004

AbstractA mathematical model describing the sulfidation of a single calcined limestone particle was developed and experimentally verified. This model, which includes no fitting parameters, assumes a calcined limestone particle to consist of spherical grains of various sizes that react with H2S according to the classic shrinking‐core model. The initial size distribution of the grains is derived from mercuiy porosimetiy. The transport of H2S through the bidisperse limestone particle is calculated based on the randompore model of Wakao and Smith, which distinguishes macropore and micropore zones. Knudsen diffusivity inside the micropore zones is calculated according to the dusty‐gas approach. The single‐particle model delivers the value of a new defined utilization factor, which includes effects of external mass‐transfer limitation, pore‐diffusion limitation, and grain‐size distribution on particle reactivity. A correlation derived for a single batch of calcined limestone explicitly expresses this utilization factor as a function of conversion and relevant process parameters. This correlatiori can be easily incorporated into reactor models, as shown for an existing model describing the capture of H2S by a fluidized bed of calcined limestone particles.
Predictive control of sheet and film‐forming processes
Authors: Campbell John C. , Rawlings James B. 
Published: April 2004

AbstractA constrained infinite‐horizon model‐predictive controller is advocated to improve product properties from sheet‐ and film‐forming processes by feedback control; the proposed model structure captures the main characteristics of the process, including the scanning sensor and large time delay. Model parameters are identified from process data, and symmetry is enforced to improve the parameter estimates. The film thickness is estimated from sparse measurements using a periodic Kalman filter. The covariances of the estimates are used to evaluate scanning patterns. Integrating disturbance models are used to achieve offset free control. Detectability conditions are presented for both models to ensure that the disturbances can be estimated from the measurement. Once the state of the film is reconstructed adequately from the measurement, a control strategy can be implemented to improve film properties. One important control contribution is to enable the controller to handle hard constraints on the actuators without clipping the controller output signal. The target‐tracking problem is discussed, and the uniqueness of the steady‐state solution is established. By simulations an output disturbance model is compared with a measured input disturbance model.
Effect of the shear and volume fraction on the aggregation and breakup of particles
Authors: Serra Teresa , Casamitjana Xavier 
Published: April 2004

AbstractThe aggregation and breakup of particles in a Couette flow system were studied for different values of the shear rate (G) and the volume fraction of particles ϕo. After a certain time, an initial distribution of 2‐μm‐dia. latex particles achieves the steady state, reflecting the balance between coagulation and fragmentation forces. Higher G values shift the steady state to smaller aggregate sizes. For a fixed value of G, when ϕo increases, the steady‐state size increases if the flow is laminar and decreases if the flow is turbulent. The population balance model proposed describes simultuneous coagulation and fragmentation during shear flocculation. A power dependence between the breakage rate coefficient and the shear rate was found. The model is adjusted by using two parameters: the probability of success for collisions α and the effective breakup coefficient for shear fiagmentation B'eff depends on both G and ϕo, while α is nearly constant.
Reliable computation of homogeneous azeotropes
Authors: Maier Robert W. , Brennecke Joan F. , Stadtherr Mark A. 
Published: April 2004

AbstractIt is important to determine the existence and composition of homogeneous azeotropes in the analysis of phase behavior and in the synthesis and design of separation systems, from both theoretical and practical standpoints. A new method for reliably locating any and all homogeneous azeotropes for multicomponent mixtures is presented. The method also verifies the nonexistence of homogeneous azeotropes if none are present. The method is based on interval analysis, in particular an interval‐Newton/generalized‐bisection‐algorithm providing a mathematical and computational guarantee that all azeotropes are located. This general‐purpose technique can be applied in connection with any thermodynamic models. The technique is illustrated in several example problems using the Wilson, NRTL, and UNIQUAC activity‐coeficient models.
Visualization of the UF polarized layer by holographic interferometry
Authors: Fernández‐Torres M. J. , Ruiz‐Beviá F. , Fernández‐Sempere J. , López‐Leiva M. 
Published: April 2004

AbstractThe technique of holographic interferometly used to visualize the ultrafiltration membrane surroundings makes it possible to visualize solute retention and verify whether or not there is cake formation on the membrane surface during the ultrafiltration of bovine serum albumin (BSA) protein using a polyethersulfone membrane. The experimental results indicate the presence of a gelatinous mass or a filter cake during BSA ultrafiltration using the membrane mentioned. This was shown by the small concentration profile obtained from the interferometric information and from direct observation of the membrane environs when the pump stopped, which showed the appearance of a gelatinous material adjacent to the membrane.
Polymorphism of CaCO3, precipitated in a constant‐composition environment
Authors: Tai Clifford Y. , Chen F.‐B. 
Published: April 2004

AbstractPolymorphism of calcium carbonate was investigated using a constant‐composition method, and three polymorphs—calcite, aragonite, and vaterite—are produced. Factors that affect the formation of polymorphs are studied over a wide range, including solution pH, temperature, concentration ratio of components, supersaturation, ionic strength, and type and concentration of additives. At room temperature, solution pH is the most significant factor, and high yield polymorphs are present at different pH values. At a high temperature, aragonite becomes the major product at a pH below 12. The effects of the additive are complicated, depending on its type and concentration as well as the operating conditions. The influence of concentration ratio and supersaturation on the formation of polymorphs is less significant, Besides, various particle morphologies of CaCO3 polymorphs are observed, depending on the operating conditions.
Process systems and inventory control
Authors: Farschman Chad A. , Viswanath Krishnan P. , Erik Ydstie B. 
Published: April 2004

AbstractA framework for modeling constraints on the dynamic behavior of systems obeying the laws of thermodynamics is discussed. These constraints induce dissipativity and we call a system obeying such constraints a process system. Passivity theory then shows that low‐dimensional macroscopic analysis can be used for control system design. The inputs and outputs converge to their set points and the state vector converges to a stationary passive state provided one exists. Several examples are developed to support the theory and an extension is proposed for systems with equipment constraints.