摘要:

AbstractThe phenomenon of colloidally induced fines migration is a challenge of both scientific and industrial importance. Its occurrence impacts permeability reduction and alteration of flow pathways in porous media, particulate contaminant migration in groundwater flow, and filtration. The release of smectitic fines is a threshold type of process resulting from discontinuous jumps, called microquakes in the interlayer spacing. There is a critical salt concentration at which these microquakes occur and produce fines migration in the porous media. The changes in the microstructure with decreasing salt concentration were analyzed using X‐ray diffraction. The transition between crystalline and osmotic swelling regime is also shown to depend on the type of cation. A mathematical model developed using colloidal principles predicts the swelling behavior of smectites in aqueous solutions. The model can predict the transition of swelling from crystalline to osmotic regimes and explain the effect of different cations on the transitio

ISSN:0001-1541    

DOI:10.1002/aic.690430302

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractComplex mixing flows and mixing parameters are calculated to evaluate mixing quality on the basis of kinematic parameters together with a statistical analysis. The results allow for a comparative evaluation in terms of geometry and flow parameters. The evaluation is limited to two‐dimensional flows with moving boundaries in periodic motion such as the flow in the cross section of a twin‐cam mixer. The finite‐element calculation requires a remeshing procedure for every time step with special techniques to upgrade mixing variables from one time step to the next. The relative mixing quality of single cam and co: or counterrotating cam devices are compared by evaluating segregation scale, length stretch, and efficiency. The numerical results exhibit a good correspondence with their experimental counte

ISSN:0001-1541    

DOI:10.1002/aic.690430303

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractNumerical tools for the analysis of complex 3‐D mixing devices were developed. Calculations are based on the finite‐element method. The flow calculation is combined with that of pathlines for a fairly large number of material points; along these pathlines, relevant kinematic variables, which are then subjected to a statistical treatment, are evaluated. These numerical techniques are applied to the analysis of a Kenics mixer. It is assumed that the geometry is periodic in the axial direction, that is, that the mixer contains an infinite number of blades; the finite‐element analysis may then be completed on a single wavelength of the geometry. To test the accuracy of the three‐dimensional calculation, a Kenics device was mounted in order to mix clays of different colors, and photographs of experimental cross‐sections were compared with numerical results showing concentration maps. A statistical treatment was performed on some 8,000 material points crossing the entry section to evaluate mixin

ISSN:0001-1541    

DOI:10.1002/aic.690430304

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractThe traditional way of determining the slip velocity of molten polymers is the classic Mooney technique, which utilizes experimental data obtained from a capillary rheometer. However, measurements of the rheological properties of polymer melts in capillary flow at high shear rates are often complicated by viscous heating, which is not taken into account by this method. A data analysis procedure based on a mathematical model for nonisothermal capillary flow of molten polymers is developed. Conduction, convection, and viscous heating are included, together with the effect of wall slip. The technique provides detailed velocity and temperature fields in the die, and can be used to determine the slip velocity at high shear rates corrected for the effect of viscous heating. It is tested for the capillary flow of several polymers, including polystyrene, polypropylene, high‐density, and linear low‐density polyethyle

ISSN:0001-1541    

DOI:10.1002/aic.690430305

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractThe unsteady heat‐transfer processes from oblate or prolate spheroids, at the limit of very small Peclet numbers is examined. A perturbation technique for the temperature and the geometry of the particle is used to obtain the rates of heat and mass transfer, first in the Laplace and then in the time domain. A solution to the problem is obtained, including the ϵ2contribution (ϵ is the eccentricity). The solution reveals the existence of several history terms, which are analogous to the history terms of the creeping flow equation of motion. One of these terms is solely due to the eccentricity of the spheroid. This is an indication that the shape of the particle is a factor of the existence and from of history terms. In addition, an exact expression for the steady‐state heat transfer from a spheroid is obtained using a convenient transformation of the heat‐transfer i

ISSN:0001-1541    

DOI:10.1002/aic.690430306

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractResonant power absorption is an important phenomenon during microwave heating. The resonances that occur when plane electromagnetic waves are incident on infinitely long cylinders and slabs are investigated as a function of sample dimensions. For cylinders two kinds of incident waves are studied:TMzwhen the electric field is oriented along the axis of the cylinder andTEzwhen the magnetic field is oriented along the axis. At a resonant condition the overage power absorbed by the sample is a local maximum. Due to attenuation within the sample the resonances decrease in intensity as the sample size increases. Using the dielectric properties of water, resonances are found to be a function of the ratio of the sample dimension to the wavelength of radiation, λs, in the sample. For slabs of thickness L and integer values of n, resonances occurred at L/λs= 0.5n; for cylinders of diameter D, resonances occurred at D/λs= 0.5n – 0.257. The generality of these relations to predict resonances in other food samples are shown using existing dielectric data. Resonances in cylinders for both polarizations occurred at similar radii. However, the radius at which the first resonance occurred for theTMzcase was absent in theTEz

ISSN:0001-1541    

DOI:10.1002/aic.690430307

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractTransient phenomena in solid–liquid fluidized‐bed systems are important in designing pulsed, countercurrent (multistage) fluidized‐bed contactors of the Cloete‐Streat type at high‐solids flow rate. Of particular interest are the residence times or corresponding velocities of porosity gradients in the bed and the excess or overshoot height of the bed after refluidization. Theory assuming local equilibrium between holdup and velocity of the phases (local‐equilibrium model) for stepwise perturbations in the liquid flow is readily available. It is investigated whether the local‐equilibrium theory can be used for more complex perturbations and whether inertia effects, such as are encountered in countercurrent multistage fluidized‐bed systems, can be ignored. Therefore, the detailed particle‐bed model of Foscolo and Gibilaro, which incorporates inertia effects, was applied to investigate the transient behavior of fluidized‐bed systems. Transient fluidization experiments were performed with a broad range of water‐fluidized particles in a laboratory‐scale multistage fluidized‐bed contactor. The operating conditions corresponded to those for countercurrent contact.Numerical simulations with the particle‐bed model predict satisfactory experimental results. The “overshoot” heights of the fluidized bed were estimated correctly by the particle‐bed model, whereas the local‐equilibrium model only provides a conservative estimate. However, the local‐equilibrium model allows an analytical solution that is more interesting for design, as it avoids tedious calculations. The residence time of the last perturbation before the fluidized bed relaxes to steady state was estimated

ISSN:0001-1541    

DOI:10.1002/aic.690430308

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractElectrophoresis of a solute through a column in which its transport is governed by the convection – diffusion equation is described. Approximate solutions to the convection – diffusion equation in the limit of small diffusion are developed using perturbation methods. The diffusion coefficient and velocity are assumed to be functions of space and time such that both undergo a sudden change from one constant value to another within a thin transition zone that itself translates with a constant velocity. Two cases are considered: (1) the thickness ϵfof the transition zone is negligible compared to the diffusional length scale, so the zone may be treated as a singular boundary across which the diffusion constant and velocity suffer discontinuous changes; (2) the transition zone is considerably wider than the diffusional length scale, so the diffusion coefficient and velocity, although sharply varying, are smooth functions of position and time. A systematic perturbation expansion of the concentration distribution is presented for case 1 in terms of the small parameter ϵ = 1/Pe. A lowest order approximation is given for case 2. A suitably configured system analyzed here can lead to progressive accumulation, or focusing, of the transported solute. The degree of focusing in case 1 scales with ϵ−1, whereas in case 2 it scales with (ϵfϵ)−1/2, and thus increases much more weakly with increasing Pe. A separation based on this concept requires development of materials and devices that allow dynamic tuning of the mass‐transport properties of a medium. This would make it possible to achieve progressive focusing and separation of solutes, such as proteins and DNA fragments, in electrophoretic media with an unprecedented deg

ISSN:0001-1541    

DOI:10.1002/aic.690430309

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractA spontaneous, steady capillary flow is produced for a liquid index in a circular tube that is partially coated with a surface modifier to create a discontinuous wetting condition between upstream and downstream portions of the tube. As a means of demonstrating that the flow configuration may prove useful as a diagnostic tool in studies concerning capillarity and the physics associated with the moving contact line, average steady velocities are predicted and compared against a large experimental data set that includes the effects of tube dimensions and fluid properties. To access a wider range of tube diameters (0.516–9.88 mm), experiments are performed employing “U‐tubes” tested in the low‐gravity environment of a drop tower, in addition to straight capillary tubes tested horizontally in a laboratory. The sensitivity of the steady capillary flow to surface cleanliness is

ISSN:0001-1541    

DOI:10.1002/aic.690430310

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractFrequency‐domain measurements of photon migration were conducted in concentrated polystyrene suspensions to assess their use for on‐line particle sizing in undiluted process streams. Using a numerical inverse algorithm, particle‐size distributions (PSD) and volume fractions of three latex suspensions with differing distributions were recovered from phase‐shift measurements at 15 or less visible wavelengths. Comparison to dynamic light‐scattering measurements show excellent agreement. Since it is the propagation characteristics of multiply scattered light instead of the amount of light detected that is employed to solve the inverse problem, the measurement technique provides a self‐calibrating (and therefore especially suitable) method for on‐line process monitoring of PSD in the chemical‐b

ISSN:0001-1541    

DOI:10.1002/aic.690430311

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY