Particle/gas suspension

The more common approach is to treat the particle-gas suspension as an equivalent gray surface parallel to the heat transfer surface. Equation (11) would than be used with Ftaken as unity. Grace (1986) suggests that the emissivity of the particle-gas suspension can be approximated as,... 

Consider the impingement between two opposed particles-gas suspension streams from accelerating tubes of the same diameter. The assumptions made in the establishment of the model are (1) The streams are symmetrical with respect to both the jet axis and the impingement plane (2) The gas flow velocity and all the physical properties of gas and solid are kept constant and (3) The particles beyond collision penetrate into the opposing stream up to. rlllas, while any particle will be drawn out of the system immediately once it collides with another particle. 

Ciesla F. J. and Hood L. L. (2002) The nebular shock wave model for chondrule formation shock processing in a particle-gas suspension. Icarus 158, 281-293. 

Valiveti P, Koch DL (1998) Instability of Sedimenting Bidisperse Particle Gas Suspensions. Applied Scientific Reseach 58 275-303... 

Vortex dryers are characterized by a spiral flow of a particle-gas suspension due to tangential entry of the gas stream into the dryer chamber. Wet material is fed directly into the drying chamber by a screw feeder or nozzle atomizer, or dispersed with the drying medium (air, inert gas, superheated steam, etc.). After performing several spiral trajectories, the particulate material leaves the dryer with an exhaust gas. Alternatively, it may be separated from the gas within the dryer and further discharged via a rotary lock. 

Laboratory reactor for studying three-phase processes can be divided in reactors with mobile and immobile catalyst particles. Bubble (suspension) column reactors, mechanically stirred tank reactors, ebullated-bed reactors and gas-lift reactors belong the class of reactors with mobile catalyst particles. Fixed-bed reactors with cocurrent (trickle-bed reactor and bubble columns, see Figs. 5.4-7 and 5.4-8 in Section 5.4.1) or countercurrent (packed column, see Fig. 5.4-8) flow of phases are reactors with immobile catalyst particles. A mobile catalyst is usually of the form of finely powdered particles, while coarser catalysts are studied when placing them in a fixed place (possibly moving as in mechanically agitated basket-type reactors). 

In a slurry reactor (Fig 5.4.74), the catalyst is present as finely divided particles, typically in the range 1-200 pm. A mechanical stirrer, or the gas flow itself, provides the agitation power required to keep the catalytic particles in suspension. One advantage is the high catalyst utilization not only is the diffusion distance short, it is al.so possible to obtain high mass-transfer rates by proper mixing. 

We used polycrystalline films of ZnO and Sn02 as adsorbents. The films were deposited from the water suspension of respective oxides on quartz substrates. These substrates contained initially sintered contacts made of platinum paste. The gap between contacts was of about lO" cm. All samples were initially heated in air during one hour at T 500 C. We used purified molecular oxygen an acceptor particle gas. H and Zn atoms as well as molecules of CO were used as donor particles. We monitored both the kinetics of the change of ohmic electric conductivity and the tangent of inclination angle of pre-relaxation VAC caused by adsorption of above gases and the dependence of stationary values of characteristics in question as functions of concentrations of active particles. 

Fog. Any heterogeneous suspension of droplets of water or other liquid substances in a gas which disturbs its transparency may be called a fog. If the particles in suspension are solid, the fog is usually called smoke or "smog . In Chemical Warfare Service, the term "Smoke is used for both liquid and solid suspensions... 

Inside the accelerating tube the particles are accelerated by the air flow from zero to a certain velocity, usually 60-70% of the gas flow velocity. During acceleration, the relative velocity between the particles and the gas flow is very high. On the other hand, the concentration of particles in the solid-gas suspension to be processed with impinging streams is generally very small, as mentioned above, so that the interaction between particles can be neglected. Therefore the movement of particles can be described approximately with Newton s motion equations for a single particle, as follows ... 

Enyakin, Yu. P. (1984). Depth of penetration of solid or liquid phase particles in opposing gas-suspension jets. J. Eng. Physics of Fluids, 14 512-514. 

Filtration is a physical separation whereby particles are removed from the fluid and retained by the filters. Three basic collection mechanisms involving fibers are inertial impaction, interception, and diffusion. In collection by inertial impaction, the particles with large inertia deviate from the gas streamlines around the fiber collector and collide with the fiber collector. In collection by interception, the particles with small inertia nearly follow the streamline around the fiber collector and are partially or completely immersed in the boundary layer region. Subsequently, the particle velocity decreases and the particles graze the barrier and stop on the surface of the collector. Collection by diffusion is very important for fine particles. In this collection mechanism, particles with a zig-zag Brownian motion in the immediate vicinity of the collector are collected on the surface of the collector. The efficiency of collection by diffusion increases with decreasing size of particles and suspension flow rate. There are also several other collection mechanisms such as gravitational sedimentation, induced electrostatic precipitation, and van der Waals deposition their contributions in filtration may also be important in some processes. 

As it is well known, the contacts between drops (in emulsions), solid particles (in suspensions) and gas bubbles (in foams) are accomplished by films of different thickness. These films, as already discussed, can thin, reaching very small thickness. Observed under a microscope these films reflect very little light and appear black when their thickness is below 20 nm. Therefore, they can be called nano foam films. IUPAC nomenclature (1994) distinguishes two equilibrium states of black films common black films (CBF) and Newton black films (NBF). It will be shown that there is a pronounced transition between them, i.e. CBFs can transform into NBFs (or the reverse). The latter are bilayer formations without a free aqueous core between the two layers of surfactant molecules. Thus, the contact between droplets, particles and bubbles in disperse systems can be achieved by bilayers from amphiphile molecules. 

For small catalyst particles completely different reactor types are used. The catalyst is now suspended in the flowing reaction mixture and has to be separated at the reactor exit or is carried along with the fluid. Particle sizes are now from 10 fim up to 1 mm. In Figure 1.2 some of the common reactor types are shown. Figure 1.2(a) shows the fluid bed reactor, where the gaseous feed keeps the small catalyst particle in suspension. Catalyst carried over in the exit stream is separated, for example, in cyclones. For even shorter contact times, riser reactors are used in which the solid catalyst is transported in the gas stream. Fluid bed reactors are also used for feed mixtures of a liquid and a gas. 

Azbel and Liapis (1983) analyzed gas/liquid systems with the assumption that the available energy at steady state is at a minimum. Reh (1971) mentioned the concept of the lowest resistance to fluid flow, and in a somewhat alternate way, the so-called minimum pressure drop was used by Nakamura and Capes (1973) in analyzing the annular structure in dilute transport risers. The instability of a uniform particle-fluid suspension was analyzed by introducing small disturbances into the system (Jackson, 1963 Grace and Tuot, 1979 Batchelar, 1988). 

Just as the critical gas velocity is required to suspend the particles in a three-phase fluidization in an agitated reactor, some minimum agitation intensity is required to keep the particles in suspension. Calderbank11 has described the methods of estimating this minimum agitation intensity. For an agitated liquid-solid slurry vessel, the correlation of Zweitering150 for the calculation of the minimum impeller speed which completely suspends the particles, namely... 

In gas suspensions, where very fine particles have to be removed, US action involves agglomeration of particles in order to increase their size and, consequently, to improve the collection efficiency of conventional filters (e.g. electrostatic precipitators, cyclone separators). These filters, while effective for large particle separation, are inefficient for retaining particles smaller than 2.5 pm. Therefore, acoustic agglomeration provides a means for separating fine particles released from industrial, domestic or vehicle sources, which, analytically, constitutes an excellent method for sampling in environmental analysis. 

Aerosols, this means aerosol dispensers, are any non-refillable receptacles made of metal, glass or plastics and containing a gas compressed, liquefied or dissolved under pressure, with or without a liquid, paste or powder, and fitted with a release device allowing the contents to be ejected as solid or liquid particles in suspension in a gas, as a foam, paste or powder or in a liquid state or in a gaseous state. 

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