摘要:

AbstractCoalescence of electrolytically‐generated, 50‐ to 600– μm‐diameter gas bubbles was observed using an optical technique that employs a linear photodiode array to detect interface movement with a resolution of 10−6s. When two bubbles coalesce, the surface energy that is released causes interface velocities of 2 to 4 m/s; these are followed by large‐amplitude damped oblate–prolate oscillations. Within the viscosity range studied, the oscillation period is insensitive to the viscosity and damping is insensitive to surface tension, in agreement with a scaling analysis based on a damped harmonic oscillator. Surface waves are superimposed on the motion. Finite‐element solutions of the free‐surface, nonlinear Navier‐Stokes equations are consistent

ISSN:0001-1541    

DOI:10.1002/aic.690431002

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractDirect numerical simulations were used to investigate turbulence structure and scalar transfer across a shear‐free, nonwavy gas‐liquid interface (free surface) in stably stratified turbulent flows. These simulations solve the Navier–Stokes and thermal‐energy conservation equations, using a finite difference approximation. The relation between the turbulence structure in the stably stratified turbulent boundary layer and organized motion near the free surface are discussed. In addition, scalar transfer across a free surface is investigated by solving a passive scalcr conservation equation after both velocity and temperature fields are fully developed. The effects of stable stratification on the scalar transfer process in a region very close to the free surface are also di

ISSN:0001-1541    

DOI:10.1002/aic.690431003

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractDilute suspensions of neutrally buoyant, uniform‐diameter spheres in a viscous liquid are sampled through pores in the wall of a stirred tank. These studies are performed to investigate screening effects near the sampling orifice that cause samples to be deficient in solids. The ratio of sphere concentration in the sample to that of the original suspension ranges from 0.08 to 1.12. Sample concentration decreases with decreasing sampling pore diameter and increasing stirring rates. The objectives of this work are both to obtain experimental relationships and to develop predictive techniques relating flow fields to sample concentration.Sample concentration is predicted by calculating particle trajectories in the region near the pore and by accounting for steric exclusion. Particle trajectories can differ from fluid streamlines due to lift forces generated in the flow field. These estimates constitute a priori predictions, since they do not rely on or introduce any empirical parameters. Predicted values of sample concentrations agree closely with dat

ISSN:0001-1541    

DOI:10.1002/aic.690431004

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractThe miscible displacement of aqueous lignin solutions (model black liquor) by water from beds formed from glass beads was studied as a function of the presence of cationic polymer in the wash water, bed structure, lignin concentration, and superficial flow rate. The displacement behaviors in homogeneous beds formed from fine beads (121 μm) were compared with results of experiments using a channel bed consisting of a bed formed from fine beads surrounding a channel of coarse beads (638 μm) running the length of the bed in the flow direction. Washing efficiencies, defined as the fraction of lignin removed when one pore volume of eluate was collected, were 84–93% for homogeneous beds washed with water compared with 31–33% for the channel bed; the range in efficiencies reflects the influence of other variables. The presence of cationic polymer in the wash water enhanced the washing efficiency by 1.7 to 2 times with a corresponding 20 to 35% decrease in the permeability of the central channel in the channel bed. The improved washing with polymer was due to selective plugging of the central channel with precipitate formed from complex formation between anionic lignin in the black liquor and the cationic polymer. Breakthrough curves obtained from 12 microconductivity probes located throughout the bed showed that mixing of lignin in the black liquor and the cationic polymer. Breakthrough curves obtained from 12 microconductivity probes located throughout the bed showed that mixing of lignin solution (high conductivity) with wash water (low conductivity) in the displacement front, as expressed by a mixing length, was a maximum in the coarse bead channel and was decreased when the wash water contained cationic po

ISSN:0001-1541    

DOI:10.1002/aic.690431005

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractA laser Doppler anemometer was used to measure velocity components for the flow of a Newtonian fluid in a reversing section of a pilot‐scale corotating twin‐screw extruder. Measurements were made at a fixed axial position in the intermeshing (nip) region and the translational region (áway from the nip). Tangential and axial velocity profiles reached a maximum in the middle of the screw channel, indicating pressure flow. Leakage flow through the gaps between screw flights and backflow toward the hopper were observed only in the nip region. No reversal of flow was detected in the translational region. Measured velocity profiles showed substantially higher shear rates in the reversing section of the screw as compared to those in the forward‐conveying s

ISSN:0001-1541    

DOI:10.1002/aic.690431006

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractThe phase holdups and the heat‐transfer behavior were studied experimentally in three‐phase fluidized beds over a pressure range of 0.1–15.6 MPa. Bubble characteristics in the bed are examined by direct flow visualization. Pressure effects on the bubble coalescence and breakup are analyzed mechanistically. The study indicates that the pressure affects the hydrodynamics and heat‐transfer properties of a three‐phase fluidized bed significantly. The average bubble size decreases and the bubble‐size distribution becomes narrower with an increase in pressure. The bubble‐size reduction leads to an increase in the transition gas velocity from the dispersed bubble regime to the coalesced bubble regime, an increase in the gas holdup, and a decrease in the liquid and solids holdups. The pressure effect is insignificant above 6 MPa. The heat‐transfer coefficient between an immersed surface and the bed increases to a maximum at pressure 6–8 MPa and then decreases with an increase in pressure at a given gas and liquid flow rate. This variation is attributed to the pressure effects on phase holdups and physical properties of the gas and liquid phases. A mechanistic analysis revealed that the major heat‐transfer resistance in high‐pressure three‐phase fluidized beds resides in a liquid film surrounding the heat‐transfer surface. An empirical correlation is proposed to predict the heat‐transfer coefficient und

ISSN:0001-1541    

DOI:10.1002/aic.690431007

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractIn cyclones, the boundary layer formed on the collecting‐wall surface acts as a barrier for particle migration toward the wall due to a decreased centrifugal force on particles inside the boundary layer. A new theory for high‐efficiency cyclones based on the boundary‐layer characteristics is presented. The cyclone was divided into two regions: the turbulent‐core region where the centrifugal force is large, and the near‐wall region where the centrifugal force is small. Particle trajectories in the turbulent‐core region are calculated from the mean fluid motion based on the quasi‐steady drag assumption, and the collection probability of particles in the near‐wall region is calculated by the deposition velocity that results from both turbulent diffusion and centrifugal force. The deposition velocity by centrifugal force was assumed equal to the equilibrium migration velocity at a certain point inside the boundary layer, and the distance to that point from the wall is assumed to be linearly proportional to the dimensionless‐particle relaxation time. When the proportional constant was determined by fitting the theoretical results to experimental data, the theory showed an excellent enhancement in predicting the variation of collection efficiency with the inlet flow velocity

ISSN:0001-1541    

DOI:10.1002/aic.690431008

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractThe isotope exchange technique (IET) can be used to simultaneously measure multicomponent gas adsorption equilibria and self‐diffusivities of the components in a single isothermal experiment without disturbing the overall adsorbed phase. An experimental protocol for the IET and corresponding data analysis procedures is described. Isotherms and self‐diffusivities for adsorption of N2as a pure gas were measured on commercial samples of a carbon molecular sieve and a 4‐Å zeolite using IET, as well as those of O2and N2from their binary mixtures. The carbon molecular sieve did not exhibit thermo‐dynamic selectivity for air separation, but had a kinetic selectivity of O2over N2Mass‐transfer resistances for self‐diffusion of N2and O2on the carbon molecular sieve were controlled by pore mouth restrictions in the carbon, but those for adsorption of N2into the 4‐Å zeolite by Fickian diffusion inside the adsorbent. A linear driving force model described the uptakes of N2and O2in the carbon molecular sieve. The Fickian diffusion model described the N2uptake in the 4‐Å zeolite. Mass‐transfer coefficients for both O2and N2on the carbon molecular sieve increased linearly with increasing gas‐phase partial pressure of these gases, and the pressure of O2did not affect mass‐transfer coefficients for N2. The self‐diffusivity of N2in the 4‐Å zeolite decreased with

ISSN:0001-1541    

DOI:10.1002/aic.690431009

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractThe adsorption selectivity, Si, j= (Xi/yi)/(xj/yj), is examined in a 1–D model of a mixture of hard rods adsorbing on a linear substrate. The adsorbed components are characterized by their length, li, and equilibrium constant, ki. We derive an expression for the bulk‐phase pressure Po, for which there is no separation of components i and j, S,i, j= 1, in terms of these parameters. The selectivity of the smaller component always increases with the bulk pressure, which results in a selectivity reversal if, as is usually the case, the larger component is more strongly adsorbed at low surface coverages. In a binary mixture, when PPo, the reverse is true. The effect of varying the molecular parameters on the selectivity is also st

ISSN:0001-1541    

DOI:10.1002/aic.690431010

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractA fundamental model based on chemical reaction kinetics and diffusional mass transfer for the extraction of zinc sulfate with di(2‐ethyhexyl) phosphoric acid (D2EHPA) in n‐heptane at 250C was developed. Gradients of the chemical potential were used as the driving force for diffusion. Activity coefficients and their derivatives were calculated from the Pitzer equation in the aqueous phase, while the organic non‐ideality was considered by the Hildebrand–Scott treatment. The Nerst–Planck equation was chosen for describing the diffusion of aqueous ions, paying special care to the formation of hydrogen sulfate. It was assumed that this fast ionic reaction can be described in terms of the mass action law in the bulk and film. For the multicomponent mass transfer of the organic species, the Maxwell–Stefan theory was adopted. A kinetic equation for the extraction of zinc with D2EHPA, which considers the adsorption of the D2EHPA molecules at the interface based on the Langmuir law, was suitable for our experimental data. Organic zinc concentration vs. time was obtained in a type of Lewis cell with vibrational mixing. Molar fluxes were calculated by fitting it with rational functions, which were analytically differentiated. Initial conditions of the experiments cover a wide range of the zinc (0.1 mM to 0.05 M), D2EHPA, and sulfuric acid concentration. Experimental fluxes could be well described by this model when chemical kinetics and mass transfer were considered. In particular, the resistance to mass transfer in the organic film i

ISSN:0001-1541    

DOI:10.1002/aic.690431011

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY