ISSN:0001-1541    

DOI:10.1002/aic.690370702

出版商:American Institute of Chemical Engineers    

年代:1991

数据来源: WILEY

摘要:

AbstractThe onset of three‐dimensional reaction‐driven convection in a porous medium is investigated using linear stability theory. The geometries investigated include a finite cylinder and a rectangular parallelepipped of arbitrary aspect ratios. The analysis determines, among other things, the likely modes (flow patterns) to emerge first as a function of reaction parameters and aspect ratios. The flow fields corresponding to three‐dimensional modes are described in detail. Important qualitative differences are found between reaction‐driven convection and the standard Lapwood or Bénard convection due to a temperature gradient applied to the boundaries of the system.The second part of the work examines numerically reaction‐driven natural convection in a porous two‐dimensional rectangular box. Orthogonal collocation and continuation techniques are used to determine the conduction and convection branches of solutions as a function of the Rayleigh number (Ra), the Frank‐Kamenetskii number (δ) and the aspect ratio (α). The convective solutions (streamlines and isotherms) corresponding to primary, secondary and tertiary bifurcations are presented. The effect of natural convection (Ra) on the ignition point (critical δ value) is determined for three differ

ISSN:0001-1541    

DOI:10.1002/aic.690370703

出版商:American Institute of Chemical Engineers    

年代:1991

数据来源: WILEY

摘要:

AbstractNonlinearities give the opportunity to influence the time average performance of a system by periodic forcing of external parameters. In this article, this concept is applied to the following reactions occurring in a continuous stirred tank reactor: A → B, A → C; A ⇋ B; A → B → C; and A → B, A + B → C. It is shown, using a generalized II‐criterion, that if the activation energies satisfy the constraint E1>E2, then the yield of B is improved using high‐frequency periodic perturbations in the temperature regardless of reaction order and operating conditions. Conversely, if E1>E2, then high‐frequency forcing always leads to a deterioration of the yield. There exist regions of parameter space where low‐frequency forcing may lead to an improvement. Results valid for low and intermediate frequencies are developed and can be used to predict additional areas of parameter space of interest f

ISSN:0001-1541    

DOI:10.1002/aic.690370704

出版商:American Institute of Chemical Engineers    

年代:1991

数据来源: WILEY

摘要:

AbstractA new type of multiphase catalyst is developed for transport‐controlled, volatile‐reactant‐limited reactions: the tubular‐supported ceramic membrane. A microporous, catalytically‐impregnated γ‐Al2O3film (permeable membrane) coats the inside wall of a hollow, macroporous α‐Al2O3tube. The gas containing the volatile reactant flows through the tube core, and the liquid containing the nonvolatile and dissolved volatile reactants flows on the shell side. Reaction occurs in the film. Results using the hydrogenation of α‐methylstyrene to cumene on Pd/Al2O3reveal that the tubular catalyst compares favorably to conventional catalyst designs. The results demonstrate that the main benefit is the more efficient supply of the volatile reactant. Apparent activation energy analyses reveal that for catalyst temperatures below 40° C, the rate is kinetically‐controlled and catalyst utilization nearly complete. A reduction in the activation energy above 45°C indicates the onset of transport limitations or heat effects. A comparison between the tubular catalyst and a fully‐wetted pellet reveals rate increases

ISSN:0001-1541    

DOI:10.1002/aic.690370705

出版商:American Institute of Chemical Engineers    

年代:1991

数据来源: WILEY

摘要:

AbstractA computational study of fully‐developed flow of gas‐particle suspensions in vertical pipes was carried out, using the model proposed recently by Sinclair and Jackson (1989), to understand the predicted scale‐up characteristics. It was shown that the model can capture the existence of steady‐state multiplicity wherein different pressure gradients can be obtained for the same gas and solids fluxes. A pronounced and nonmonotonic variation of the pressure gradient required to achieve desired fluxes of solid and gas with tube diameter was predicted by the model, and this is explained on a physical basis. The computed results were compared with the experimental data. The model manifests an unsatisfactory degree of sensitivity to the inelasticity of the particle‐particle collisions and the damping of particle‐phase fluctuating motion

ISSN:0001-1541    

DOI:10.1002/aic.690370706

出版商:American Institute of Chemical Engineers    

年代:1991

数据来源: WILEY

摘要:

AbstractAxial dispersion coefficients have been measured in single‐phase flow conditions in a 5.08‐cm‐dia. reciprocating plate extraction column. The measurements were done under steady‐state conditions by analyzing temperature profiles (for hot and cold water mixing) and concentration profiles (for mixing of water and salt solutions). The results confirm previous published data of Holmes et al. (1991) in showing that axial dispersion is increased strongly in the unstable situation, where liquid density increases with height. The earlier work is extended to the condition where buoyant energy dissipation is exceeded by mechanical energy dissipation by four orders of magnitude. Even in this case, axial mixing is increased significantly by density gradient‐induced instability. Results have been correlated by a simple equation based on a mixing len

ISSN:0001-1541    

DOI:10.1002/aic.690370707

出版商:American Institute of Chemical Engineers    

年代:1991

数据来源: WILEY

摘要:

AbstractA theoretical model comprising the processes of micropore, macropore and surface diffusion was derived and applied to the dynamics of adsorption of CO2on a single activated‐carbon particle. The effects of a nonlinear isotherm (Langmuir), particle nonisothermality and concentration dependence of the surface diffusivity were also incorporated. The theoretical model was fitted to a preliminary set of adsorption experimental data (two particle sizes, and various temperatures and concentrations) to determine the diffusion coefficients. The validity of the theoretical model was checked by using the model, with fitted diffusivities, to simulate dynamic adsorption curves at conditions different from those of the preliminary fitting data set. These model simulations were found to predict the experimental data in a satisfactory manne

ISSN:0001-1541    

DOI:10.1002/aic.690370708

出版商:American Institute of Chemical Engineers    

年代:1991

数据来源: WILEY

摘要:

AbstractThis work examines reaction‐induced flow maldistributions in adiabatic, downflow packed beds. Using linear stability analysis it is shown that for the case of a constant heat source, the uniform flow loses stability when a certain dimensionless group, the Darcy buoyancy number, exceeds a critical value. Center manifold theory is used to analyze the local bifurcation picture for the case of a simple and double zero eigenvalue. It is found that for large Peclet numbers, all the bifurcations from the uniform solution are subcritical in nature and are unstable locally. Orthogonal collocation and continuation techniques are combined with the local theory to determine the various branches of bifurcating solutions. The temperature and flow distributions of stable and unstable solution branches are presented for several aspect ratios and Peclet numbers. Numerical simulations predict direct transitions from uniform flow to periodic or chaotic flows. It is also found that there is a wide range of the Darcy buoyancy number in which the uniform and maldistributed flows are stable and coexis

ISSN:0001-1541    

DOI:10.1002/aic.690370709

出版商:American Institute of Chemical Engineers    

年代:1991

数据来源: WILEY

摘要:

AbstractThe kinetics of particle coarsening are studied by means of contour dynamics simulation in two dimensions. The current method is free from the usual restrictions, such as small area fraction, constant particle shape, and local particle motion. Growth or shrinkage of particles in a given realization is found to depend on the local environment of the particles. The concept of critical size central to the classical theories such as the LSW theory, therefore, appears to be inappropriate. Surprisingly, the ensemble‐averaged macroscopic behavior is consistent with mean‐field theory. The particle‐size distribution (PSD) tends toward an asymptotic shape, different from the mean field result. The normalized width of the PSD seems to be insensitive to volume fraction. Interfacial transport limitation can be significant for small particles, and it has the effect of altering the width of the PSD. The coarsening rate constant is found to be a function of the area fra

ISSN:0001-1541    

DOI:10.1002/aic.690370710

出版商:American Institute of Chemical Engineers    

年代:1991

数据来源: WILEY

摘要:

AbstractAn internal model control (IMC) strategy for nonlinear single‐input single‐output systems is proposed. The controller is designed to provide nominal performance, and a nonlinear filter is added to make the controller implementable and to account for plant/model mismatch. An important advantage of the new approach is that the assumption of full‐state feedback inherent in most input‐output linearization schemes is eliminated. However, the proposed IMC strategy is restricted to open‐loop stable systems with stable inverses. Under mild assumptions, the closed‐loop system possesses the same stability, perfect control, and zero offset properties as linear IMC. Simulation results for a continuous fermentor illustrate the advantages of the nonlinear I

ISSN:0001-1541    

DOI:10.1002/aic.690370711

出版商:American Institute of Chemical Engineers    

年代:1991

数据来源: WILEY