摘要:

AbstractAs steady‐state and dynamic simulators with optimal controllers have evolved, tools for nonlinear analysis have been developed by reactor and thermophysical property specialists. Designers lag behind in developing and utilizing these tools, as many design strategies assume idealized behavior even though processes contain an increasing number of nonlinear elements. This article examines the sources of the nonlinear elements, often associated with physical and chemical interactions, in designs and their control systems. Promising methods for nonlinear analysis are reviewed.In each design activity, involving simulation, economic optimization, projections of controllability and operability, emphasis is on the sources of nonlinearities and solution methods. These are examined for the equality constraints and nonlinear programs in phase and chemical equilibria, in reactors and separators, and in the simulation and optimization of flowsheets. Also examined are the impact of non‐linear predictive control in screening designs and methods to insure operability, in the face of uncertaint

ISSN:0001-1541    

DOI:10.1002/aic.690370102

出版商:American Institute of Chemical Engineers    

年代:1991

数据来源: WILEY

摘要:

AbstractSelective oxidation kinetics of n‐butane to maleic anhydride in air were studied over a commercial, fixed‐bed vanadium‐phosphor oxide catalyst. The temperature range was 573–653 K with butane concentrations up to 3 mol % in the feed, which is within flammability limits but below ignition temperatures.The rate data were modeled using power law kinetics with product inhibition and included total oxidation and decomposition reactions. Kinetic parameters were estimated using a multiresponse, nonlinear regression algorithm showing intercorrelation effects. The kinetics were combined with independent measurements of catalyst diffusivity and reactor heat transfer using a one‐dimensional heterogeneous reactor model. Model predictions and observed temperatures and concentrations from non‐isothermal pilot plants were compared up to 115 days on stream. Agreement was acceptable with inlet butane concentrations up to 2.7 mol %. For example, runaway was predicted at a salt temperature 3 K higher than observed. Effectiveness factors around the hot spot were estimated at 0.6 with the catalyst surface temperature 2–3 K higher than the average ga

ISSN:0001-1541    

DOI:10.1002/aic.690370103

出版商:American Institute of Chemical Engineers    

年代:1991

数据来源: WILEY

摘要:

AbstractA new model that interrelates the liquid‐phase excess Gibbs energy, enthalpy and volume, called the GHV model, is developed and discussed. The model is based on the fact that most mixtures are immiscible in solid phases and, therefore, have steep solid‐phase boundaries. This single assumption allows the model to be applied potentially to all types of mixtures, including organic, metallic, electrolytic and polymeric solutions. The model, which predicts that excess properties are related to one another through various fusion properties, pure component volumes, and enthalpies of fusion in particular, has been applied to a wide variety of organic and liquid metal mixtures. The results show at least qualitative validity without adjustable parameters and more quantitative ability with “effective” fusion properties. The model is also shown to rationalize some sign, magnitude and shape ph

ISSN:0001-1541    

DOI:10.1002/aic.690370104

出版商:American Institute of Chemical Engineers    

年代:1991

数据来源: WILEY

摘要:

AbstractAn analytical solution is found for flow in a converging channel for Reynolds numbers in the range of 102to 103. The solution applies to an axisymmetric channel, for which the shape is given by R2z=constant, where R is the channel radius at the axial distance z. This shape produces a center‐line or core velocity that increases linearly with z, thus making the extensional rate constant. The analytical solution is a pseudosimilarity solution of the axial momentum equation, and its accuracy was gauged by comparison to other results. Comparisons of velocity and pressure distributions to experimental data and to finite‐element results indicate that the analytical solution is accurate to about

ISSN:0001-1541    

DOI:10.1002/aic.690370105

出版商:American Institute of Chemical Engineers    

年代:1991

数据来源: WILEY

摘要:

AbstractCommon models for excess Gibbs energy account primarily for physical forces and do not represent strong specific interactions. However, purely chemical models neglect strong nonspecific forces and combined chemical‐physical treatments involve an unreasonable number of adjustable parameters. One way to ameliorate this limitation is to make independent measurements of the parameters by a nonther‐modynamic technique. We report an investigation of hydrogen bonding, a localized and very directional specific interaction, conducted in various alcohol mixtures using Fourier transform nuclear magnetic resonance to measure the number of complexed species in solution. A chemical‐physical association model with variable equilibrium constants has been developed. The distinguishing feature of this model is that the species in solution do not have equal probability of forming yet require only a single chemical interaction parameter determined from fitting NMR data and a physical interaction parameter determined from infinite dilution activity coefficients. The AVEC model successfully predicts vapor‐liquid equilibrium and excess enthalphy as well as liquid‐liquid equilibrium, an especially strin

ISSN:0001-1541    

DOI:10.1002/aic.690370106

出版商:American Institute of Chemical Engineers    

年代:1991

数据来源: WILEY

摘要:

AbstractEffective Knudsen diffusion coefficients are presented for fibrous structures consisting of overlapping fibers. The fibers are distributed randomly in d (d=2 or 3) directions with their axes perpendicular to one direction (d=2) or in the three‐dimensional space with no preferred orientation (d=3), or they are grouped into d (d=1, 2, or 3) mutually perpendicular bundles of parallel, randomly overlapping fibers. Effective diffusivities are computed using a Monte Carlo simulation scheme to determine the mean square displacement of molecules traveling in the interior of the porous medium for large travel times. Our results show that structures with fibers distributed randomly in d directions have diffusion coefficients identical, within the accuracy of our simulation, to those of d‐directional, parallel fiber structures. Effective Knudsen diffusivities are strongly influenced only by the directionality of the fiber structure, with tridirectional or randomly oriented fiber structures presenting lower percolation thresholds (0.04 vs. 0.11) and higher effective diffusivities than bidirectional or random structures with their axes perpendicular to one direction. The tortuosity factor is in general found to decrease with increasing porosity, approaching for each case, as the porosity goes to unity, the corresponding lower bound that is derived using variational princip

ISSN:0001-1541    

DOI:10.1002/aic.690370107

出版商:American Institute of Chemical Engineers    

年代:1991

数据来源: WILEY

摘要:

AbstractMicrokinetic simulations have been carried out to describe the partial oxidation of methane over silica‐supported molybdena and vanadia. The objective of this study was to formulate a physically reasonable reaction network to capture the basic aspects of the surface catalytic chemistry and to use this network to link methane oxidation kinetics with the kinetics observed for the oxidation of subsequent gaseous products, i.e., methanol, formaldehyde, and carbon monoxide. The most abundant reactive intermediates on the catalyst surface are suggested to be oxygen and hydroxyl groups. This reaction network successfully predicts the catalytic activities, selectivities, activation energies, and reaction orders observed over silica‐supported molybdena and vanadia; and the microkinetic model also semiquantitatively explains the kinetic trends for related catalyst systems and is consistent with surface bonding energetics reported in the literature over various oxide catalysts. The microkinetic reaction network properly reduces to the more simple macrokinetic model reported previously for methane partial oxidat

ISSN:0001-1541    

DOI:10.1002/aic.690370108

出版商:American Institute of Chemical Engineers    

年代:1991

数据来源: WILEY

摘要:

AbstractFluid‐mediated interaction between catalyst particles alone is shown to yield several interesting and significant phenomena in a catalytic reactor that have been generally attributed in the past to direct interaction between particles. Thus, collaborative interaction between particles and the fluid may enhance or abate steady‐state multiplicity, and reverse stability behavior. From the simple setting of a population of particles in a well‐mixed CSTR, it is shown that the catalyst phase in a catalytic reactor is susceptible to very fine pattern formation in the face of steady‐state multiplicity in single particles, which negates the usual assumption that particles exposed to a given fluid have identical states. In a reactor such variability in behavior must be accompanied by a corresponding variability in conversion and selectivity (in multireaction systems) and may have strong implications for reactor control str

ISSN:0001-1541    

DOI:10.1002/aic.690370109

出版商:American Institute of Chemical Engineers    

年代:1991

数据来源: WILEY

摘要:

AbstractThe time‐averaged void fraction, pressure drop and flow regime transition behavior of horizontal air‐water two‐phase flows is studied experimentally and numerically for 2‐cm‐inner‐diameter tubes with various flow dividing junctions at its end. The time‐average void and pressure drop behavior along the channel is simulated using a two fluid separated flow model. The results show that two‐phase behavior (flow regime, void fraction, and pressure drop) is affected strongly by the presence of a flow division in the system. These effects extend far upstream of the junction for low‐momentum flows and far downstream for high‐momentum flows. Both numerical and experimental results show that there occurs a large increase in void just downstream of the junction owing to the halving of the fluid volume flow rates and the

ISSN:0001-1541    

DOI:10.1002/aic.690370110

出版商:American Institute of Chemical Engineers    

年代:1991

数据来源: WILEY

摘要:

AbstractA detailed linear and nonlinear analysis of void wave phenomena in two‐phase flow is presented. A state‐of‐the‐art, one‐dimensional two‐fluid model was used as the basis for this analysis. It is shown that void wave dispersion is strongly influenced by phasic slip. In particular, the smaller the slip the more dispersion occurs. It has also been found that nonlinear wave forms, such as shocks and solitons, may occur. Moreover, the virtual mass force was found to be an important parameter in the modeling of void waves. It is shown that void wave analysis is an excellent means of assessing the closure assumptions used in two‐fluid models. It is also implied that carefully taken void wave data are needed for

ISSN:0001-1541    

DOI:10.1002/aic.690370111

出版商:American Institute of Chemical Engineers    

年代:1991

数据来源: WILEY