Mechanical particle transport

Mechanisms of Mechanical Particle Transport. When particles do not follow, but diverge from, airflow streamlines and thereby come in contact with airspace surfaces, particle deposition occurs. This diverging from airflow streamlines and particle trajectories is mainly due to mechanisms of mechanical particle transport inertial, gravitational, and diffusional particle transport (Fig. 3). The... 

In physical terms, particles are classified into different domains relative to their predominant mechanism of mechanical particle transport. Particles smaller than 0.1 pm are related to the thermodynamic domain and particles larger than 1 pm to the aerodynamic domain. A transitional domain is defined for those particles with diameters between 0.1 and 1 pm (10). A detailed overview of the different particle classifications is given in Chapter 1. 

The simplest binary valued CA proven to be computation universal is John Conway s two-dimensional Life rule, about which we will have much to say later in this chapter. Many of the key ingredients necessary to prove universality, however, such as sets of propagating structures out of which analogs of conventional hardware components (i.e., wires, gates and memory) may be explicitly constructed, appear, at least in principle, to be supported by certain one-dimensional rules as well. The most basic component required is a mechanism for transporting localized packets of information from one part of the lattice to another i.e., particle-like persistent propagating patterns, whose presence is usually indicative of class c4 behavior. 

Frequency of Collisions between Particles. Particles in suspension collide with each other as a consequence of at least three mechanisms of particle transport ... 

Very little is known of the mechanism(s) by which the rate of degradation is controlled. The current view is that the concentrations of ubiquitin, together with changes in the activity of the proteasome complex control the rate of proteolysis by this system. Lysosomal degradation may be controlled by the number of particles transported into the cell. The calpains might be controlled by the ion concentration. 

Although the term acid rain has been used extensively in the popular literature to describe the formation and deposition of acids at the earth s surface, the terminology acid deposition is more commonly encountered in the scientific literature. The reason for this is that deposition of acids can occur either as dry deposition or as wet deposition. The former refers to the direct transport of acidic gases or small particles to the surface, followed by adsorption, without first being dissolved in an aqueous phase such as rain, clouds, or fog. Wet deposition, on the other hand, refers to the transport of acids to, and deposition on, surfaces (including soil, trees, grass, buildings, etc.) after the acids have been dissolved in an aqueous medium. It should be noted that the surface itself can be either wet or dry the terms wet and dry deposition refer to the mechanism of transport to the surface, not to the nature of the surface itself. 

These layers containing higher concentrations of pollutants provide an important mechanism for transport of ozone, particles, and their precursors to the free troposphere. In addition, in the morning when solar heating causes turbulent mixing (Fig. 2.20), these pollutants are mixed down to the surface. This not only increases the surface concentrations but also provides species that can initiate the VOC-NO chemistry that leads to more ozone formation. As a result, there is a carryover from one day to the next, leading to smog episodes in which the pollutant concentrations increase from day to day. 

FI G U RE 10.2 Schematic representation of alveolar cells and possible mechanism of transport of molecules from the alveolar space into the circulation. Particles will release molecules of interest (gray circles) into the mucus in which the particle is embedded. The molecule can either be lost in the mucus, taken up by alveolar macrophages by phagocytosis or diffusion, taken up by alveolar epithelial cells by passive or active transport, or bypass the alveolar cells via paracellular transport depending upon the properties of the drug. Once a molecule has reached the extracellular space, the same mechanisms are possible for transport from the extracellular space into the blood. Molecules in the extracellular space may also reach to circulation via the lymph. 

The plasma lipoproteins are spherical macromolecular complexes of lipids and specific proteins (apolipoproteins or apoproteins). The lipoprotein particles include chylomicrons, very-low-density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). They differ in lipid and protein composition, size, and density (Figure 18.13). Lipoproteins function both to keep their component lipids soluble as they transport them in the plasma, and also to provide an efficient mechanism for transporting their lipid contents to (and from) the tissues. In humans, the transport system is less perfect than in other animals and, as a result, humans experience a yradual deposition of lipid—especially cholesterol—in tissues. This is a potentially life-threat-en ng occurrence when the lipid deposition contributes to plaque formation, causing the narrowing of blood vessels (atherosclerosis). 

The hydrodynamics are mainly influenced by the size and form of the particles and by the particle size distribution, which in most cases should be in a range between 0.4 and 0.8 mm. Too small particle sizes can cause channelling which, for example, can occur during the extraction of paprika powder, egg-yolk powder, cocoa powder, and algae powder. Fibrous feed materials such as ginger, black- and green-tea, and paprika have a tendency to swell and block the filters at the outlet of the extractor. The mechanism of transport in the solid phase proceeds in the following parallel and consecutive steps. 

An extremely effective means of enhancing heat removal from a reactor is to make use of fluidized-bed technology (3). Heat transfer coefficients for gaseous systems are increased to values of around 600 W/m2K or more by virtue of the very efficient convective-regenerative particle transport mechanism of heat transfer. Further... 

In the development above, it has been convenient to consider collisions, via particle transport, and reactions, the probability of particle attachment, as separate steps. There are a number of considerations indicating that this conceptual framework may have outlived its usefulness, as advancements in particle science, analytical capabilities, and supercomputers obviate the necessity of this artificial separation. Adler [4], Han and Lawler [3], and others have demonstrated using trajectory modeling the significant influence of hydrodynamics on particle collisions, and show how the lumped collision efficiency (inclusive of hydrodynamics) is a function of the type of collision mechanism. Thus, for a given particle pair, the collision efficiency will be different depending on whether the collision is a result of Brownian motion, fluid shear, or differential sedimentation. 

Sintering of supported solids can occur by two distinct mechanisms particle migration and coalescence as already mentioned above and interparticle transport of atoms or molecules (Ostwald ripening)... 

One successM theory of hansport processes in liquids are based on elementary acts, each act consisting of two steps (1) holes are formed and (2) particles jump into these holes (see Section 5.7.4). For fused salts and other nonassociated liquids, this theory was successful in explaining the movements and drift of particles although it clashed with molecular dynamics calculations that seemed to favor a shuffle-along mechanism for transport. The mean volume of a hole is determined by the surface tension as follows [cf. Eq. (5.44)] ... 

It is the purpose of this paper to study which mechanism, particle-to-particle or vapor phase transport, is responsible fa- the mobility of vanadium in the FCC unit The approach used in this work is to measure the rate of vanadium transport to a basic oxide "vanadium trap" in fluid bed experiments. By varying the particle size distribution, the collision frequency can be changed and the rate of transport determined. Also, calculations of the mass transfer of a vapor species in a fluid are performed. 

There is an apparent paradox from these experiments vanadium moves readily from catalyst to trap in a fluid bed, yet vanadium transpiration is negligible under the same conditions. This result would suggest that the mechanism of transport is by particle-to-particle collisions in the bed. Still, an alternate question arises, what are the requirements for mass transfer of vapor phase vanadium in a fluid bed Following the correlation of Richardson and Szekely (72), the mass transfer coefficient, km, in a fluid bed can be calculated from the Sherwood number, Sh, and a knowledge of the particle Reynolds number. Re, in the bed by. 

Particles smaller than 0.1 tim diameter are able to diffuse through the laminar boundary layer by Brownian diffusion, the efficiency of the mechanism increasing as particle size decreases below 0.1 (J-m. In general, rates of Brownian diffusion, which are small even by comparison with molecular diffusion, and do not therefore, represent an efficient process for the transport of sulphur and nitrogen containing particles across the laminar boundary layer. Another mechanism for transport of particles through this layer is inertial impaction. For this process the particle must... 

---