摘要:

AbstractA molecular‐based statistical simulation program was developed to study the covariance terms involved in the mass balance equations for complex chemical reactions during mixing. Several closure theories were compared to the simulations and available experimental data. The simple closure by Brodkey and Lewalle was found to be an extension of Toor's analysis applied to two reactions. This closure does not satisfy the molar fluctuation balance equation and was found only to represent the high Reynolds number data of Li and Toor. This result led to examining other possible closures which were based on Damkoehler numbers, reaction rate constant ratios, and limiting forms of the covariance term. These closures also were inadequate. The second reaction's covariance term varied from the product of the average values for each component to the Brodkey and Lewalle value for the range of Reynolds numbers considere

ISSN:0001-1541    

DOI:10.1002/aic.690361002

出版商:American Institute of Chemical Engineers    

年代:1990

数据来源: WILEY

摘要:

AbstractThis paper concerns general MIMO nonlinear processes, whose dynamic behavior is described by a standard state‐space model of arbitrary order, including measurable disturbances. The concept of relative order of an output with respect to an input, extended to include disturbance as well as manipulated inputs, is generalized in a MIMO context and it is used to obtain a characterization of the dynamic interactions among the input and the output variables. A synthesis formula is calculated for a feedforward/state feedback control law that completely eliminates the effect of the measurable disturbances on the process outputs and induces a linear behavior in the closed‐loop system between the outputs and a set of reference inputs. The input/output stability and the degree of coupling in the closed‐loop system are determined by appropriate choice of adjustable parameters. A MIMO linear controller with integral action completes the feedforward/feedback control structure. The developed control methodology is applied to a continuous polymerization reactor and its performance is evaluated through simula

ISSN:0001-1541    

DOI:10.1002/aic.690361003

出版商:American Institute of Chemical Engineers    

年代:1990

数据来源: WILEY

摘要:

AbstractA Phenomenological model is proposed for the rates of bubble coalescence and bubble break‐up in turbulent gas‐liquid dispersions. Bubble coalescence is modeled by considering bubble collisions due to turbulence, buoyancy, and laminar shear, and by analysis of the coalescence efficiency of collisions. Bubble break‐up is analyzed in terms of bubble interactions with turbulent eddies. A method for the measurement of coalescence and break‐up events in turbulent systems is described and used to test the validity of the proposed model. The measurement technique relies on the mixing of tracer gases within bubbles upon coalescence, in conjunction with Monte‐Carlo simulations of coalescence events. Both distilled water and salt solutions are examined. Favorable agreement is found between the model and the individual coalescence and breakage rates, as well as with data obtained for the average bubble size and bubble size dis

ISSN:0001-1541    

DOI:10.1002/aic.690361004

出版商:American Institute of Chemical Engineers    

年代:1990

数据来源: WILEY

摘要:

AbstractMass transfer resistances in zeolite NaCaA after different hydrothermal pretreatment are probed by NMR tracer desorption studies with the short‐chain‐length hydrocarbons from methane ton‐butane and with xenon. In all cases, the measurements lead to the coinciding result of forming surface barriers, which differ only slightly depending on the pretreatment modes, but lead to a retardation of molecular mass transfer more dramatically than the starting material. The dramatic retardation of mass transfer after hydrothermal pretreatment is confirmed by supplementary uptake measurements withn

ISSN:0001-1541    

DOI:10.1002/aic.690361005

出版商:American Institute of Chemical Engineers    

年代:1990

数据来源: WILEY

摘要:

AbstractA thermodynamic framework for the analysis of gas sorption in glassy polymers is established in the limit of low sorption levels. The results represent an extension of the Flory‐Huggins theory to materials with nonzero internal energy changes due to deformation; the Flory‐Huggins theory is recovered in the limit of zero polymer bulk modulus. The sorption isotherm is expressed in terms of the penetrant vapor‐phase activity, polymer physical properties, the penetrant partial molar volume, and the heat of mixing. The qualitative form of the isotherm is shown to be unrelated to the presence of excess free volume in the polymer; only quantitative predictions are influenced. The downward curvature of the isotherm is due to the thermodynamics of solid deformation. Explicit relationships between the parameters of this work and the dual‐mode model are given for the low‐sorption/high‐bulk mo

ISSN:0001-1541    

DOI:10.1002/aic.690361006

出版商:American Institute of Chemical Engineers    

年代:1990

数据来源: WILEY

摘要:

AbstractA theoretical approach to predict the solubility of proteins in solutions containing nonionic polymers is presented. The effective protein‐protein interaction due to the presence of the polymer is related to the volume‐exclusion potential of Asakura and Oosawa. Statistical‐mechanical perturbation theory, as originally applied by Gast et al. to model colloidal flocculation, is used to calculate free energies, from which solubility curves for varying protein‐polymer diameter ratios are obtained. The theory correctly predicts all the trends observed in experimental studies of these systems. To explain the influence of processes parameters such as the pH and the ionic strength on protein solubility, the intermolecular potential is improved by the addition of an electrostatic interaction term. It is found that theoretical predictions of the variation in protein solubility, both with the solution pH and the ionic strength, are in accordance with experimental obser

ISSN:0001-1541    

DOI:10.1002/aic.690361007

出版商:American Institute of Chemical Engineers    

年代:1990

数据来源: WILEY

摘要:

AbstractThree‐phase fluidized beds have played important roles in various areas of chemical and biochemical processing. The characteristics of such beds are highly stochastic due to the influence of a variety of phenomena, including the jetting and bubbling of the fluidizing medium and the motion of the fluidized particles. A novel approach, based on the concept of fractals, has been adopted to analyze these complicated and stochastic characteristics. Specifically, pressure fluctuations in a gas‐liquid‐solid fluidized bed under different batch operating conditions have been analyzed in terms of Hurst's rescaled range (R/S) analysis, thus yielding the estimates for the so‐called Hurst exponent,H. The time series of the pressure fluctuations has a local fractal dimension ofdFL= 2 −H. AnHvalue of ½ signifies that the time series follows Brownian motion; otherwise, it follows fractional Brownian motion (FBM), which has been found to be the case for the three‐phase fluidized bed

ISSN:0001-1541    

DOI:10.1002/aic.690361008

出版商:American Institute of Chemical Engineers    

年代:1990

数据来源: WILEY

摘要:

AbstractHomogeneous nucleation is achieved in aqueous systems by creating adequately high levels of supersaturations. This process can be fully described by considering depletion of supersaturation due to simultaneous growth of the nuclei. Nielsen (1964) formulated this by integrating the growth kinetics with classical nucleation theory. This paper uses his combined nucleation‐growth approach, as well as more recently developed mechanisms of adding monomers to developing nuclei (Chiang et al., 1988). The experimental data considered suggest that the systems fall into two categories: one involving the same mechanism for nucleation and growth and the other involving different mechanisms. A dimensionless parameter which evolves in the development permits the comparison of the rate of nuclei formation with that of nuclei growth. The inferences from this approach suggest further that the surface energy parameter is an indicator of the category to which a solution system belong

ISSN:0001-1541    

DOI:10.1002/aic.690361009

出版商:American Institute of Chemical Engineers    

年代:1990

数据来源: WILEY

摘要:

AbstractAll classical models for thickening assume one‐dimensional continuity. However, the free‐settling domain in continuous thickeners is not one‐dimensional. Therefore, a two‐dimensional model is investigated. The two‐dimensional model gives the same values for thickener area demands as the one‐dimensional model, but the relationships between batch and steady‐state thickening are not the same. Free‐settling Kynch characteristics can arise in the continuous operation that do not arise in batch tests. Therefore, design procedures that rely on Kynch theory, such as that of Talmage and Fitch, are not completely valid.A method is developed, based on extrapolating the Kynch or free‐settling segment of a batch settling, curve, that yields an improved prediction for thic

ISSN:0001-1541    

DOI:10.1002/aic.690361010

出版商:American Institute of Chemical Engineers    

年代:1990

数据来源: WILEY

摘要:

AbstractA reaction engineering model has been developed to describe the mercury‐sensitized photochemical vapor deposition of hydrogenated amorphous silicon (a‐Si:H) semiconductor thin films. Model equations governing the gas‐phase generation, transport, and surface reactions of SiH3and H film precursor radicals are solved to predict film growth rate and bonded hydrogen content. Behavior of the model has been studied as a function of deposition conditions (pressure, temperature, feed composition, and flow rates) and has been verified by comparison with experimental re

ISSN:0001-1541    

DOI:10.1002/aic.690361011

出版商:American Institute of Chemical Engineers    

年代:1990

数据来源: WILEY