Particle collectors

Fig. 22. Performance cut diameter predictions for typical dry packed bed particle collectors as a function of bed height or depth, packing diameter and packing porosity (void area) S. Bed irrigation increases collection efficiency or decreases cut diameter (271). SoHd lines, = 25 mm dashed lines,...

A wide variety of special-purpose incinerators (qv) with accompanying gas scmbbers and soHd particle collectors have been developed and installed in various demilitarisation faciUties. These include flashing furnaces that remove all vestiges of explosive from metal parts to assure safety in handling deactivation furnaces, to render safe small arms and nonlethal chemical munitions fluidized-bed incinerators that bum slurries of ground up propellants or explosives in oil and rotary kilns to destroy explosive and contaminated waste and bulk explosive. 

Peters, J. M., Predicting Efficiency of Fine-Particle Collectors, Chem. Engineering, April 16, 1973, p. 99. 

Stokes law, 226, 230 Stokes-Cunningham law, 226, 230 Dust, mist, particle collector performance chart, 229... 

In filtration, the particle-collector interaction is taken as the sum of the London-van der Waals and double layer interactions, i.e. the Deijagin-Landau-Verwey-Overbeek (DLVO) theory. In most cases, the London-van der Waals force is attractive. The double layer interaction, on the other hand, may be repulsive or attractive depending on whether the surface of the particle and the collector bear like or opposite charges. The range and distance dependence is also different. The DLVO theory was later extended with contributions from the Born repulsion, hydration (structural) forces, hydrophobic interactions and steric hindrance originating from adsorbed macromolecules or polymers. Because no analytical solutions exist for the full convective diffusion equation, a number of approximations were devised (e.g., Smoluchowski-Levich approximation, and the surface force boundary layer approximation) to solve the equations in an approximate way, using analytical methods. 

Similar to chemical vapor deposition, reactants or precursors for chemical vapor synthesis are volatile metal-organics, carbonyls, hydrides, chlorides, etc. delivered to the hot-wall reactor as a vapor. A typical laboratory reactor consists of a precursor delivery system, a reaction zone, a particle collector, and a pumping system. Modification of the precursor delivery system and the reaction zone allows synthesis of pure oxide, doped oxide, or multi-component nanoparticles. For example, copper nanoparticles can be prepared from copper acetylacetone complexes [70], while europium doped yttiria can be obtained from their organometallic precursors [71]. 

Gas evaporation using Ar for the preparation of various sort of metal fine powders was first reported by Kimoto et al. in 1963 (5). The production chamber of this method is basically the same as that of a vacuum sublimation chamber. A target material is heated in this chamber with several torr inert gas atmosphere. The nanometer-sized particles are easily formed in the chamber space. However, by this method, it is difficult to get genuine nanoparticles whose sizes are several nanometers. This is because of the radiation heating in a production chamber, resulting particle coalescence on the chamber wall or particle collector, as well as the direct particle contact in the deposited particle layer (powders). Therefore the size becomes several tens to hundreds of nanometers. Several ultrafme metallic powders are now commercially available, including Cu, Ag, Al, Ni, Co, Fe, and Au, with a size of several tens of nanometers. 

The collection of particles is achieved in a countercurrent flow between the water droplets and the particulates. In a cyclonic scrubber, water is injected into the cyclone chamber from sprayers located along the central axis, as shown in Fig. 7.19. The water droplets capture particles mainly in the cross-flow motion and are thrown to the wall by centrifugal force, forming a layer of slurry flow moving downward to the outlet at the bottom of the cyclone. Another type of scrubber employs a venturi, as shown in Fig. 7.20. The velocity of the gas-solid suspension flow is accelerated to a maximum value at the venturi throat. The inlet of the water spray is located just before the venturi throat so that the maximum difference in velocity between droplets and particles is obtained to achieve higher collection efficiency by inertial impaction. A venturi scrubber is usually operated with a particle collector such as a settling chamber or cyclone for slurry collection. 

Leith, D. and Licht, W. (1972). The Collection Efficiency of Cyclone Type Particle Collectors A New Theoretical Approach. AlChE. Symp. Ser., 68(126), 196. 

When particles change their direction of movement, as for example around bluff bodies such as cylinders or bends in tubing, inertial forces tend to modify their flow paths relative to the suspending gas. Particles may depart from the path of gas molecules (streamlines) and collide with the larger body (Fig. 2). This is the principle underlying inertial particle collectors. 

Physical adsorption arises bom physical interactions between the suspended particles and die collector, such as van der Waals attraction, double-layer repulsion, and Born repulsion. The total interaction energy, as a function of the particle-collector gap width, displays either one minimum and no maximum or two minima and one maximum. Several mechanisms for chromatographic sep-... 

When van der Waals and double-layer forces are effective over a distance which is short compared to the diffusion boundary-layer thickness, the rate of deposition may be calculated by lumping the effect of the particle-collector interactions into a boundary condition on the usual convective-diffusion equation. This condition takes the form of a first-order irreversible reaction (10, 11). Using this boundary condition to eliminate the solute concentration next to the disk from Levich s (12) boundaiy-kyersolution of the convective-diffusion equation for a rotating disk, one obtains... 

If the double-layer repulsion is sufficiently strong to cause the total potential energy of interaction to assume a maximum (larger than kT) in its dependence on the particle-collector separation Jt, Ruckenstem and Prieve (10) have shown that the apparent rate con-... 

This corresponds to mass-transfer limitation of the apparent surface reaction. Thus the combination of Eqs. p] and 2] is expected to give reasonable estimates of the rate of deposition for all particle-collector interaction profiles, provided the interactions are confined to a region which is thin compared to the diffusion boundary layer. 

A simplified particle-trajectory analysis has been used to explain the essential features of US-assisted filtration. Acoustic forces can cause particle trajectories to deviate from hydrodynamic pathlines towards particle collectors, thereby enhancing their collection efficiency in comparison to pure hydrodynamic interception. 

Americium-241 is widely used in smoke detectors. The radiation released by this element ionizes particles that are then detected by a charged-particle collector. The half-life of 24IAm is 432.2 years, and it decays by emitting alpha particles. How many alpha particles are emitted each second by a 5.00-g sample of 241Am ... 

Having a model that has a good theoretical basis, that has been validated in laboratory experiments, and that is consistent with field observations, it is advisable to make some predictions about particle deposition in systems of interest. An example is presented in Figure 3, adapted from the work of Tobiason (1987). The travel distance in an aquifer required to deposit 99% of the particles from a suspension is termed Lgg and is plotted as a function of the diameter of the suspended particles for two different values of a(p, c), specifically 1.0 (favorable deposition) and 0.001 (deposition with significant chemical retardation of the particle-collector interaction, termed unfavorable deposition ). Assumptions include U = 0.1 m day"1, T= 10°C, dc = 0.05cm, e = 0.40, pp= 1.05 gem"3, and H=10 2OJ. These results indicate the dependence of the kinetics of deposition on the size of the particles in suspension that has been predicted and observed in many systems. Small particles are transported primarily by convective Brownian diffusion, and large particles in this system are transported primarily by gravity forces. A suspended particle with a diameter of about 3 /im is most difficult to transport. Nevertheless, in the absence of chemical retardation, a travel distance of only about 5 cm is all that is needed to deposit 99% of such particles in a clean aquifer, that is, an aquifer that has not received and retained previous particles. 

SPR biosensors are devices that are suitable for analysis of aqueous samples. Therefore, in order to detect target analytes in different real-world matrices (e.g., tissue, meat, soil, and air) the analyte has to be transferred to a liquid by a sample preparation unit. Numerous sample pretreatment methods for gas, solid, and crude liquid samples compatible with SPR biosensors are available. For detection in gas environments such as air, real-time trapping of analyte into an aqueous solution is possible by using collectors such as a wetted-wall cyclone particle collector [1]. Several optical biosensors have been integrated with these collectors and installed on aerial vehicles for real-time detection... 

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