ISSN:0001-1541    

DOI:10.1002/aic.690390102

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractWe investigated the effect of concentration on the Brownian diffusion of uncharged rigid spheres. Monosize silica spheres were prepared according to the method of Stöber (1968). The particles were sterically stabilized by chemisorption of stearic alcohol at their surface by the method developed by van Helden (1981).Particle radius was 14.5 nm from electron micrographs of the coated particles. Osmotic pressure measurements of the sterically stabilized particles dispersed in cyclohexane showed that the particles behaved as hard spheres. The measurements agreed well with predictions from the Carnahan‐Starling equation over the concentration range 0.0458<ϕ<0.37 where ϕ is the volume fraction of the particles in the suspension. Viscosity measurements of silica dispersions were made over the concentration range 0<ϕ<0.25. The relative viscosity over the range 0<ϕ<0.2 was fitted by ηr= 1 + 2.4ϕ + 7.1ϕ2. The coefficients 2.4 and 7.1 in this equation are in good agreement with the theoretical values of 2.5 and 6.2 obtained by Einstein(1906) and Batchelor(1977), respectively. The Brownian diffusion coefficient of the particles dispersed in cyclohexane was measured over the concentration range 0.0055<ϕ<0.248 using Taylor's hydrodynamic stability method. A laser fiber‐optic system was used to measure the transient concentration profile along the capillary as indicated by a fluorescent dye. This technique offers the advantage of being direct and nonintrusive. The experimental diffusivity data were found to be well described by the generalized Stokes‐Einstein equation(Batchelor, 1976) over the entire concentration

ISSN:0001-1541    

DOI:10.1002/aic.690390103

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractBrownian and turbulent diffusive deposition of submicron aerosol particles from pipe flow is studied experimentally and theoretically. A theoretical model for evaluating the turbulent diffusive deposition is presented in which a turbulent flow in a circular pipe is numerically calculated based on the k‐ϵ turbulent flow model and deposition velocities are derived by solving the convection‐diffusion equation. Deposition velocities of monodisperse aerosol particles, 0.01–0.04 μm in diameter, are obtained experimentally by measuring the decrease in the particle number concentration of an aerosol at two cross‐sections of a circular test pipe through which the aerosol is flowing. The deposition velocities obtained when Re is larger than about 3,000 agree well with those predicted by the present analysis which are proportional to the 0.92nd power of Reynolds number and the 0.33rd power of Schmidt number. The particle deposition rates are measured when 1, 000 ≲ Re ≲ 2,000 suggest a transitional state for particle deposition which cannot be explained by the present analysis nor by the laminar pipe flow depo

ISSN:0001-1541    

DOI:10.1002/aic.690390104

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractThere is as yet no theoretically sound and empirically proven procedure that is fully reliable for the design of thickeners. From a logical standpoint, a sound procedure is one correctly deduced from valid premises or assumptions. As noted in Fitch's earlier work(979), there is no reason to believe that existing mathematical models are based on uniformly valid assumptions. There is also much reason to believe that they are not. Therefore existing mathematical models or “theories” of thickening are analyzed for mathematical consistency, scientific credibility, and relevance to chemical engineering. Domains in which the models are useful approximations of reality and thus are relevant to chemical engineering practice are identified. Emendations or alternatives are suggested to bring the models more nearly into conformity with empirical results. Design procedures are suggested that, although involving assumptions that cannot be uniformly valid, would seem more reasonable than existing o

ISSN:0001-1541    

DOI:10.1002/aic.690390105

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractExperiments are reported in which the drag of single liquid droplets settling in a tall column of another lighter immiscible liquid are measured. The experimental data for the eight pairs of liquids that were tested covered a range of droplet Reynolds numbers from 10−3to 104. Two regimes of droplet settling were encountered. In the first regime, the droplets remained spherical, and the drag agreed very well with established solid sphere drag models. In the second regime, the droplets became deformed and oscillated; the drag was found to depart suddenly from predictions of spherical models and to increase with increasing Reynolds number. Empirical models for the point of departure from spherical drag and the coefficient of drag in the unstable regime are derive

ISSN:0001-1541    

DOI:10.1002/aic.690390106

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractThe increasing complexity of process plants and their reliability have necessitated the development of more powerful methods for detecting and diagnosing process abnormalities. Among the underlying strategies, analytical redundancy and knowledge‐based system techniques offer viable solutions. In this work we consider the adaptive inclusion of analytical redundancy models (state and parameter estimation modules) in the diagnostic reasoning loop of a knowledge‐based system. This helps overcome the difficulties associated with each category. The design method is a new layered knowledge base that houses compiled/qualitative knowledge in the high levels and process‐general estimation knowledge in the low levels of a hierarchical knowledge structure. The compiled knowledge is used to narrow the diagnostic search space and provide an effective way of employing estimation modules. The estimation‐based methods that resort to fundamental analysis provide the rationale for a qualitatively‐guided reasoning process. The overall structure of the fault detection and isolation system based on the combined strategy is discussed focusing on the model‐based redundancy methods which create the low levels of the hierarchical knowledge base. The system has been implemented using the condensate‐feedwater subsystem of a coal‐fired power plant. Due to the highly nonlinear and mixed‐mode nature of the power plant dynamics, the modified extended Kalman filter is used in designing local

ISSN:0001-1541    

DOI:10.1002/aic.690390107

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractA Wave‐Net is an artificial neural network with one hidden layer of nodes, whose basis functions are drawn from a family of orthonormal wavelets. The good localization characteristics of the basis functions, both in the input and frequency domains, allow hierarchical, multiresolution learning of input‐output maps from experimental data. Furthermore, Wave‐Nets allow explicit estimation for global and local prediction error‐bounds, and thus lend themselves to a rigorous and explicit design of the network. This article presents the mathematical framework for the development of Wave‐Nets and discusses the various aspects of their practical implementation. Computational complexity arguments prove that the training and adaptation efficiency of Wave‐Nets is at least an order of magnitude better than other networks. In addition, it presents two examples on the application of Wave‐Nets; (a) the prediction of a chaotic time‐series, representing population dynamics, and (b) the classification of experimental data for process

ISSN:0001-1541    

DOI:10.1002/aic.690390108

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractThis article presents an approach to fault diagnosis of chemical processes at steadystate operation by using artificial neural networks. The conventional back‐propagation network is enhanced by adding a number of functional units to the input layer. This technique considerably extends a network's capability for representing complex nonlinear relations and makes it possible to simultaneously diagnose multiple faults and their corresponding levels in a chemical process. A simulation study of a heptane‐to‐toluene process at steady‐state operation shows successful results for the proposed a

ISSN:0001-1541    

DOI:10.1002/aic.690390109

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractThe occurrence of instabilities in chemically reacting systems, resulting in unsteady and spatially inhomogeneous reaction rates, is a widespread phenomenon. In this article, we use nonlinear signal processing techniques to extract a simple, but accurate, dynamic model from experimental data of a system with spatiotemporal variations. The approach consists of a combination of two steps. The proper orthogonal decomposition [POD or Karhunen‐Loève (KL) expansion] allows us to determine active degrees of freedom (important spatial structures) of the system. Projection onto these “modes” reduces the data to a small number of time series. Processing these time series through an artificial neural network (ANN) results in a low‐dimensional, nonlinear dynamic model with almost quantitative predictive capabilities.This approach is demonstrated using spatiotemporal data from CO oxidation on a Pt (110) crystal surface. In this special case, the dynamics of the two‐dimensional reaction profile can be successfully described by four modes; the ANN‐based model not only correctly predicts the spatiotemporal short‐term behavior, but also accurately captures the long‐term dynamics (the attractor). While this approach does not substitute for fundamental modeling, it provides a systematic framework for processing experimental data from a wide variety of spatiotemporally varying reaction engin

ISSN:0001-1541    

DOI:10.1002/aic.690390110

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractPorous particles of two commercially available ZnO sorbents differing in porosity, surface area, and pore‐size distribution were reacted with H2S at 500 and 600°C in a fixed‐bed reactor. Concentration breakthrough curves were determined by analyzing the effluent of the reactor using a gas chromotograph equipped with thermal conductivity and flame photometric detectors. The pore structure of samples collected from different positions in the reactor was analyzed by mercury porosimetry and gas adsorption to determine the variation of the average structural properties of the sorbent with the length of the reactor. The obtained experimental data were used to validate a fixed‐bed desulfurization model, which employs detailed submodels for diffusion, reaction, and structure evolution in the porous sorbent particles. With the various parameters appearing in the submodels determined from independent thermogravimetric reactivity evolution experiments, the fixed‐bed desulfurization model was found to be capable of providing an excellent description of the behavior of the desulfurization sorbents in a fixed‐b

ISSN:0001-1541    

DOI:10.1002/aic.690390111

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY