Multiphase particles

In addition to yielding particle size and size distribution, particle volume, particle mass and particle density (Le. the properties on which sqtarations are based), SFFF can so provide information on the thickness of adsorbed layers such as surfactants, information on complex multiphase particles such as in core-shell latexes (especially when coupled with other diagnostic techniques) and can also address the important problem of agglomerates. 

D. Snider, An incompressible three-dimensional multiphase particle-in-cell model for dense particle flows. Journal of Computational Physics 170 (2) (2001) 523-549. 

SongF, Li F, Wang W, LiJ Multiphase particle-in-ceU simulation offluidization with a subgrid EMMS drag. 2015 (in preparation). 

Schuler B, Baumstark R, Kirsch S, Pfau A, Sandor M, Zosel A. Structure and properties of multiphase particles and their impact on the performance of architectural coatings. Prog Org Coat 2000 40 139-150. 

Core-and-shell composite particles based on inorganic cores with a polymer shell have also been investigated by several researchers, but do not seem to have reached industrial products. The reason for this is probably the high cost and possibly limited benefits of this type of latexes compared to existing products. A similar type of product is composite particles based on pre-emulsified polymers such as epoxies or polyesters (alkyds) with a subsequent addition of new monomers and polymerisation. This technique is partly connected to the process of miniemulsion polymerisation described in Section 1.2.2. A type of core-and-shell particles or at least multiphase particles may be obtained in this type of process. However, industrial applications of this type of products are not found on a large scale yet. Applications of polymer particles, mainly made by emulsion polymerisation, in the biomedical field was concentrated initially in the areas of blood flow determination and in vitro immunoassays. Microspheres have been employed for the determination of myocardial, cerebral and other blood flow and perfusion rates. Polymer particles and lattices, in particular, have been extensively used in immunoassays, starting in 1956, with the development... 

These limitations can be alleviated in the second coarse-grained approach, where the particles are stiU explicitly tracked, but collisions with neighboring particles are determined stochastically through collision prob-abihty theory. This can be combined with a parcel approach where, to save computational costs, only a small subset of all real particles are simulated. For the calculations of the collision probabihties, the nonsimulated particles are then assumed to be in the neighborhood of the simulated particles and have the same statistical properties as the local ensemble of simulated particles. Such an approach is advantageous when deaHng with extremely high numbers of particles. Two important methods that fall into this class are multiphase particle in cell (see Andrews and O Rourke, 1996 Xie et al, 2013) and DSMC (see Bird, 1976 O Rourke, 1981). Primarily because of our own experience with the method, here we will focus on DSMC. 

Andrews MJ, O Rourke PJ The multiphase particle-in-ceU (MP-PIC) method for dense particulate flows, Int J Multiphase Flow 22 379—402, 1996. 

Another approximation, one of the most enduring empirical correlations in multiphase systems, is the Richardson-Zaki correlation for a single particle in a suspension (3) ... 

X-ray diffraction consists of the measurement of the coherent scattering of x-rays (phenomenon 4 above). X-ray diffraction is used to determine the identity of crystalline phases in a multiphase powder sample and the atomic and molecular stmctures of single crystals. It can also be used to determine stmctural details of polymers, fibers, thin films, and amorphous soflds and to study stress, texture, and particle size. 

Di Felice, R., 1994. The voidage function for fluid-particle interaction systems. International Journal of Multiphase Flow, 20, 153-159. 

An estimation of the multiphase viscosity is a preliminary necessity for convenient particle processing. For particle-doped liquids the classical Einstein equation [20] relates the relative viscosity to the concentration of the solid phase ... 

The flow problems considered in previous chapters are concerned with homogeneous fluids, either single phases or suspensions of fine particles whose settling velocities are sufficiently low for the solids to be completely suspended in the fluid. Consideration is now given to the far more complex problem of the flow of multiphase systems in which the composition of the mixture may vary over the cross-section of the pipe or channel furthermore, the components may be moving at different velocities to give rise to the phenomenon of slip between the phases. 

There are many nonintrusive experimental tools available that can help scientists to develop a good picture of fluid dynamics and transport in chemical reactors. Laser Doppler velocimetry (LDV), particle image velocimetry (PIV) and sonar Doppler for velocity measurement, planar laser induced fluorescence (PLIF) for mixing studies, and high-speed cameras and tomography are very useful for multiphase studies. These experimental methods combined with computational fluid dynamics (CFDs) provide very good tools to understand what is happening in chemical reactors. 

Optical systems can be used in multiphase flows at a very low volume fraction of the dispersed phase. Through a refractory index matching of hquid-liquid or liquid-solid systems, it is also possible to measure at high void fractions. However, it is not possible to obtain complete refractory index matching since the molecules at the phase boundary have different optical properties than the molecules in the bulk. Consequently, it is possible to measure at a higher fraction of the dispersed phase with larger drops and particles because of the lower surface area per volume fluid. 

Velocity measurement of the dispersed phase in multiphase flow is possible using both PIV and LDV. In PIV, the particles can be masked according to size, and the velocity for each size fraction can be estimated [7]. The turbulent properties, for example, granular temperature, are more difficult to measure because of the low number of particles in the measured volume. With LDV it is also possible to obtain the velocity and size for the dispersed phase, but the turbulent properties for the dispersed phase are still difficult to measure accurately, owing to the low number of particles and also because the position of the particles is not exactly the same aU the time. 

Simulations of multiphase flow are, in general, very poor, with a few exceptions. Basically, there are three different kinds of multiphase models Euler-Lagrange, Euler-Euler, and volume of fluid (VOF) or level-set methods. The Euler-Lagrange and Euler-Euler models require that the particles (solid or fluid) are smaller than the computational grid and a finer resolution below that limit will not give a... 

Direct measurement of particle velocity and velocity fluctuations in fluidized beds or riser reactors is necessary for validating multiphase models. Dudukovic [14] and Roy and Dudukovic [28] have used computer-automated radioactive particle tracking (CARPT) to foUow particles in a riser reactor. From their measurements, it was possible to calculate axial and radial solids diffusion as well as the granular temperature from a multiphase KTGF model. Figure 15.10 shows one such measurement... 

C. T, Multiphase Flows with Droplets and Particles, CRC Press, Boca Raton, FL (1997). 

Microlevel. The starting point in multiphase reactor selection is the determination of the best particle size (catalyst particles, bubbles, and droplets). The size of catalyst particles should be such that utilization of the catalyst is as high as possible. A measure of catalyst utilization is the effectiveness factor q (see Sections 3.4.1 and 5.4.3) that is inversely related to the Thiele modulus (Eqn. 5.4-78). Generally, the effectiveness factor for Thiele moduli less than 0.5 are sufficiently high, exceeding 0.9. For the reaction under consideration, the particles size should be so small that these limits are met. 

Specimens on grids are an alternative to self-supporting disks. Specimens in the form of small particles may be supported on a thin film (e.g. an amorphous carbon film) before being placed on the grid. Thin slices of the material may be produced by an instrument known as an ultramicrotome. The principal advantage of the technique is that it leaves the chemistry unchanged, and it may be employed to create uniform thin films of multiphase material. The instrument operates by moving... 

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