摘要:

AbstractExperiments conducted quantify the macroscopic hydrodynamic characteristics of various scale 2‐D bubble columns, which include dispersed and coalesced bubble regimes characterized by two flow conditions (4‐ and 3‐region flow) with coherent flow structures. Hydrodynamic behavior is analyzed based on flow visualization and a particle image velocimetry (PIV) system. Columns operated in the 4‐region flow condition comprise descending, vortical, fast bubble and central plume regions. The fast bubble flow region moves in a wavelike manner, and thus the flow in the vicinity of this region is characterized macroscopically in terms of wave properties. In columns greater than 20 cm in width, the transition from the dispersed bubble flow regime to the 4‐ and then to 3‐region flow in the coalesced bubble regime occurs progressively with gas velocities at 1 and 3 cm/s, respectively. The demarcation of flow regimes is directly related to measurable coherent flow structures. The instantaneous and time‐averaged liquid velocity and holdup profiles provided by the PIV system are presented in light of the macroscopic flow structure in various 2‐D bubble columns. Numerical simulations demonstrate that the volume of fluid method can provide the time‐dependent behavior of dispersed bubbling flows and account for the coupling effects of pressure field and the liquid velocity on the bubble motion. Comparison of computational results with PIV results for two different bubble injector arrangement

ISSN:0001-1541    

DOI:10.1002/aic.690420202

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractA unified network model is formulated for predicting effective Fickian diffusivities of condensable vapors in porous media where capillary condensation and adsorption‐desorption hysteresis occur. The model unifies the equilibrium theory based on the poreblocking interpretation of hysteresis in the interconnected network of pores and the percolation model of mass transport in the network with randomly interspersed regions for capillary condensation and surface flow. The Bethe network is used to represent the porous medium, and the effective medium theory is employed to obtain the effective diffusivity. Using the information on the connectivity and the positions of the closure points of the hysteresis loop enclosed by the equilibrium primary adsorption and primary desorption isotherms, the concentration dependence of the effective diffusivity is predicted. The model is applied to the systems water vapor‐silica gel, water vapor‐activated alumina, and literature data. It successfully predicts the concentration dependence of the effective diffusivity in the whole range of relative pressures for systems both with and without a peak in the diffus

ISSN:0001-1541    

DOI:10.1002/aic.690420203

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractThe flows generated in vessels stirred by two Rushton impellers were investigated in two vessels of diameter (T) 100 and 294 mm with impellers of diameter D = T/3 using flow visualization, power consumption, mixing time, and 360° ensemble‐averaged and 1° angle‐resolved LDA measurement techniques. The flows depended strongly on the clearance of the lower impeller above the base of the vessel (C1), the separation between the impellers (C2), and the submergence (C3) of the upper impeller below the top of the liquid column height (H). When these distances were varied, three stable and four unstable flow patterns were observed. Comparisons between the two LDA techniques showed that while the 360° ensemble‐averaged measurements are useful for characterizing the overall flow structure and turbulence levels in the vessel, care must be exercised when interpreting such data, since in the impeller region they include periodic variations in the mean velocity in addition to the turbulent fluctuations. The trailing vortex structure and flow periodicity produced by the Rushton impellers is shown to decay significantly within a cylindrical region of height 1.2D and radius 1.0D centered around the middle of the vessel, when C1 = C2 = T/3. The turbulence structure within this region is anisotropic, while outside this region it might be considered mostly isotropic. The main flow features scaled well between the

ISSN:0001-1541    

DOI:10.1002/aic.690420204

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractThe velocity and pressure fields and the effect of wall flux on these fields in a spiral channel are presented. As fluid flows inward through a spiral channel with constant gap and permeable walls, the streamwise flux decreases while the curvature increases. Thus, by balancing the stabilizing effect of wall suction with the destabilizing effect of increasing curvature, established vortices can be maintained along the spiral channel. This approach is used to prescribe spiral geometries with different wall fluxes. Using a weakly nonlinear stability analysis, the influence of wall flux on the characteristics of Dean vortices is obtained. The critical Dean number is reduced when suction is through the inner wall only, is slightly reduced when suction is equal through both walls, and is increased when suction is through the outer wall only. The magnitude of change is proportional to a ratio of small numbers that measures the importance of the effect of curvature. In membrane filtration applications the wall flux is typically 2 to 5 orders of magnitude less than the streamwise flow. If the radius of curvature of the channel is of the order of 100 times the channel gap, the effect on the critical Dean number is within 2% of the no‐wall flux case. If the radius of curvature is sufficiently large, however, it is possible to observe effects on the critical Dean number that approach O(1) in magnitude for certain parameter range

ISSN:0001-1541    

DOI:10.1002/aic.690420205

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractA model of simultaneous heat and mass transfer presented describes drying with internal heat generation. Since a liquid expulsion phase is observed, a numerical procedure was developed to account for saturated and unsaturated zones and to model the liquid expulsion. The model was validated by a drainage experiment. An experimental rig was built to conduct microwave drying experiments in well‐controlled conditions using capillary porous body (light concrete) as test material. Two types of drying (high and low power) were distinguished, depending on whether or not boiling occurred in the sample. The heat source term in the medium was determined from the experimental results. The numerical results agree with the experimental observations in terms of drying kinetics and transfer mechanisms. This allows a very accurate description of the transport phenomena and the liquid expulsion phase associated with high‐power dry

ISSN:0001-1541    

DOI:10.1002/aic.690420206

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractA computer‐aided simulator of steady‐state two‐phase flow in consolidated porous media is developed. The porous medium is modeled as a 3‐D pore network of suitably shaped and randomly sized unit cells of the constricted‐tube type. The problem of two‐phase flow is solved using the network approach. The wetting phase saturation, the viscosity ratio, the capillary number, and the probability of coalescence between two colliding ganglia are changed systematically, whereas the geometrical and topological characteristics of the porous medium and wettability (dynamic contact angles) are kept constant. In the range of the parameter values investigated, the flow behavior observed is ganglion population dynamics (intrinsically unsteady, but giving a time‐averaged steady state). The mean ganglion size and fraction of the nonwetting phase in the form of stranded ganglia are studied as functions of the main dimensionless parameters. Fractional flows and relative permeabilities are determined and correlated with flow phenomena at pore level. Effects of the wetting phase saturation, the viscosity ratio, the capillary number, and the coalescence factor on relative permeabilities

ISSN:0001-1541    

DOI:10.1002/aic.690420207

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractMultiple diffusion reactions are frequently encountered in the modeling of heterogeneous catalytic reactors. Obtaining an accurate estimate of the yield and selectivity in such reactions is crucial for an optimal design of reactors. Due to the inadequacy of analytical techniques in handling nonuniform catalyst shapes and mixed boundary conditions, numerical techniques are often employed to compute these design parameters. Among other numerical techniques, the boundary element method (BEM) is a superior method to solve linear diffusion reaction problems. The integral nature of the BEM formulation allows for boundary‐only discretization of the particle, thus reducing the computer execution time and the data preparation effort. A boundary element algorithm is developed to solve a network of linear diffusion reactions in porous catalyst particles in two dimensions. For this purpose, a matrix of fundamental solutions is defined and derived. The developed algorithm is applied to complex reaction networks to obtain the yield of intermediates for nonregular catalyst shapes and nonuniform boundary conditions. The method can be used as a design tool to study particle scale modeling in detail and can be incorporated into an overall reactor mode

ISSN:0001-1541    

DOI:10.1002/aic.690420208

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractA mathematical model and numerical algorithm are developed to predict electrodeposition rates from a 2‐D jet of electrolyte that impinges on a flat surface. The principal situations of interest are for applied voltages that produce current densities below the limiting current. The motion is assumed to be at high speed with the jet inducing a thin laminar boundary layer on the surface; a progressively thinner concentration layer and an electrochemical double layer near the surface are accounted for. Two cases, corresponding to a submerged and unsubmerged jet, are considered. A boundary integral method is used to compute the current density along the plate in a general iterative numerical procedure coupled to the solution of the hydrodynamic, concentration and electrochemical boundary layers. The results show that relatively high deposition rates occur near the point of impingement and that altering the jet angle relative to the surface influences local electrodeposition rates significantl

ISSN:0001-1541    

DOI:10.1002/aic.690420209

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractA range of small, medium and large pore zeolite, and their modified forms are studied for their ability to separate di‐ from monobranched isomers of hexane. The separation studies are carried out using high‐temperature (250–350°C) gas chromatography. Beta(H,Ba) is found to be the most effective separator of 2,3‐dimethylbutane and 3‐methyl‐pentane and is therefore studied for its sorption capacities toward the two he

ISSN:0001-1541    

DOI:10.1002/aic.690420210

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractA novel gas purification technique called rapid pressure swing absorption (RAPSAB) was developed by integrating the best features of membrane contacting, gas‐liquid absorption, and pressure swing adsorption (PSA). In this cyclic separation process, a well‐packed microporous hydrophobic hollow‐fiber module was used to achieve nondispersive gas absorption from a high‐pressure feed gas into a stationary absorbent liquid on the module shell side during a certain part of the cycle followed by desorption of absorbed gases from the liquid in the rest of the cycle. The total cycle time varies between 20 s and upwards. Separation of mixtures of N2and CO2(around 10%) where CO2is the impurity to be removed was studied using absorbent liquids such as pure water and a 19.5% aqueous solution of diethanolamine (DEA). Three RAPSAB cycles studied differ in the absorption part. Virtually pure N2streams were obtained with DEA as absorbent demonstrating the capability of bulk separation to very high levels of purification. Numerical models developed predict the extent of purification for pure water and the DEA solution for one of the simpler cycles. Model simulations describe the observed behavi

ISSN:0001-1541    

DOI:10.1002/aic.690420211

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY