Sand particles

Capillary pressure gradients and Marongoni flow induce flow in porous media comprising glass beads or sand particles [40-42], Wetting and spreading processes are an important consideration in the development of inkjet inks and paper or transparency media [43] see the article by Marmur [44] for analysis of capillary penetration in this context. 

Stratification of the particles making up the bed, caused by the fluidization (fines on top), is not desirable. The soflds holding capacity of the bed is best utilized if the filtration flow encounters progressively finer sand particles. This is achieved in upflow filters where the fluidization due to backwash produces the correct stratification in the bed. Unfortunately, the filtration flow and the backwash take place in the same direction the disadvantage is that the washwater goes to the clean side of the filter. 

Foundry Resins. In the foundry industry, phenoHc resins are used as the binder for sand in the manufacture of sheU molds and cores. The two mating halves are joined by clamps or a bonding agent to form a sheU mold into which the molten metal is poured for castings. The sheU is formed by depositing a resin—sand mix on a hot metal pattern plate. After a certain period the pattern is inverted and the excess resin sand is removed. The sand particles are bonded by an oven cure, and the sheU is ejected from the pattern plate. 

Fluidized Bed. This reactor consists of a sand bed on which the biomass is grown. Siace the sand particles are small, a very large biomass can be developed ia a small volume of reactor. Ia order to fluidize the bed, a high recycle is required. 

After only 4 months of service, the main condenser at a large fossil utility began to perforate. Initial perforations were due to erosion-corrosion (see Case History 11.5). Small clumps of seed hairs entering the condenser after being blown into the cooling tower were caught on surfaces. The entrapped seed hairs acted as sieves, filtering out small silt and sand particles to form lumps of deposit (Fig. 6.24A and B). Immediately downstream from each deposit mound, an erosion-corrosion pit was found. 

In practice the friction factors are calculated either by integration of Eq. (4.51) or by reference to a Moody chart. This is based on Eq. (4.51) by using equivalent roughness values representing the sand particle roughness (see Table 4.3). 

Effective inlet air filtration is required to ensure satisfactory operation of the engine. The location of the unit determines the most appropriate filter system to use. Desert environments where a large amount of sand particles could be expected in the ambient air may use an automatic roll type of filter that allows new filter material to be rolled in front of the inlet without frequent shut-downs to change filters. Arctic or extremely cold locations may use pad type filters, snow hoods to prevent blockage, and exhaust recirculation to prevent icing. Filter assemblies for offshore marine environments may include weather louvers, demister pads, and barrier elements for salt and dirt removal. Screens may be u.sed for insect removal prior to filtration in areas with bug problems. 

Kato (K3) measured so-called critical gas velocities corresponding to the complete suspension of solids, and presents a graphical correlation of the results for glass spheres (diameters from 0.074 to 0.295 mm), magnetite particles (particle size from 0.038 to 0.175 mm), and sand particles (particle size 0.147 to 0.295 mm). 

In experiments with bubble-columns containing suspended sand particles with average diameter 0.12 mm, an increase in heat-transfer coefficient was observed with increasing sand concentration, maximum values of 6000 kcal/m2-hr-°C being measured for suspensions containing 50% sand (based on the liquid volume). 

Massimilla et al. (M5) measured the rate of absorption of carbon dioxide in water from a mixture of carbon dioxide and nitrogen. Used as solid phase were silica sand particles of average equivalent diameter 0.22 mm, or glass ballotini of average equivalent diameter 0.50 and 0.80 mm. Columns of 30-and 90-mm i.d. were used, and the column height was varied from 100 to 1200 mm. 

The terminal falling velocity of the sand particles in water may be taken as 0.0239 m/s. This value may be confirmed using the method given in Volume 2. 

Using sands of narrow and wide size distributions suspended in various organic liquids, Capes and Sutherland (C7) have shown that large, compact agglomerates are formed if the amount of the bridging liquid is sufficient to occupy about 44-88% of the pore space in a densely compacted bed of sand particles. The final size distribution attained represents a balance between... 

In 1962 Jottrand and Grunchard (J7) reported on mass transfer to a small rectangular nickel plate immersed in a liquid fluidized bed of sand particles. Mass-transfer rates were five to ten times higher than those measured in an open pipe flow a maximum rate was measured at a bed porosity of 0.58. Le Goff et al. (Lie) later showed that this maximum is directly related to a maximum in the average kinetic energy of the fluidized particles per unit bed volume. 

Hollow sand casting is a relatively simple process for making cast metal objects that for thousands of years was the most widely used of all casting methods. For the most basic castings, the molds are made from ordinary silica sand mixed with water so as to keep the sand particles compacted together to maintain the required hollow-shaped form. The hot, fluid... 

Sand has been treated with oil-soluble organosilicon compounds to form a hydrophobic proppant (77). A double layer resin coating has also been developed. The inner layer coating the sand particle is a cured gamma-aminopropyltriethoxvsilane - hexamethylenetetramine. The outer layer is an uncured mixture of the same two chemicals which cures within the fracture to form a consolidated permeable mass holding the fracture open (78). 

A screen with 100 U.S. mesh (149 micrometer) openings was placed at the bottom of the test column to prevent the production of coarse sand particles from the test column. To avoid injection fluid turbulence disturbing the test sand, a 7.5g layer of 20-40 U.S. mesh sand was placed on top of the test sand. All fluids except polymer solutions were filtered prior to injection. Polymer solutions were injected at 5 psia and immediately followed by aqueous fluid at 40 psig. Effluent fluids were collected and filtered through 0.45 micron paper to collect the produced fine particles. 

Glicksman and McAndrews (1985) determined the effect of bed width on the hydrodynamics of large particle bubbling beds. Sand particles with a mean diameter of 1 mm were fluidized by air at ambient conditions. The bed width ranged from 7.6 cm to 122 cm while the other cross sectional dimension remained constant at 122 cm. Most experiments were carried out with an open bed. The bubble rise velocity increased with the bed width, in the representation of bubble velocity as... 

The predicted effects of temperature on (/m/ wcrc experimentally observed by Botterill and Teoman (1980) as shown in Fig. 3. For sand particles 462 microns in diameter, they observed that Umf- decreased with temperature. For larger material (2320 microns) in the transition region in Fig. 2 (where (/ changes from increasing with temperature to decreasing with temperature), they observed an initial increase in f/ / w ith temperature at low temperatures, which was followed by a decrease in Umjras increasing temperature caused viscous effects to become dominant. 

Temperature of the fluidized bed is another parameter that could influence the heat transfer coefficient. Increasing bed temperature affects not only the physical properties of the gas and solid phases, but also increases radiative heat transfer. Yoshida et al. (1974) obtained measurements up to 1100°C for bubbling beds of aluminum oxide particles with 180 pm diameter. Their results, shown in Fig. 6, indicate an increase of over 100% in the heat transfer coefficient as the bed temperature increased from 500 to 1000°C. Very similar results were reported by Ozkaynak et al. (1983) who obtained measurements for bubbling beds of sand particles (dp = 1030 pm) at temperatures up to 800°C. 

Figure 11. Axial variation of solid concentration for fast fluidized bed of sand particles, at U— 5 m/s and Gs = 30 kg/m2 s. (From Herb, Dou, Tuzla and Chen,...

Arena et al. (1983) investigated the coal attrition in a mixture with sand under hot but inert conditions. As they increased the sand particle size while keeping its mass in the bed constant, they observed an increase in the coal attrition rate. They interpreted their results by assuming that the abrasion energy is shared out on the entire material surface. On the same basis Ray et al. (1987a) developed their attrition rate distribution model for abrasion in a fluidized bed. 

During start-up, the microbial population distribution in the biofilm varies with time. Initial colonization of the particle may be by one or more species that alter the surface favorably for colonization by other species. For instance, in the operation of a butyrate-degrading fluidized bed bioreactor, methanogens attached to the sand particles early in the start-up process and produced a primary matrix of heteropolysaccharides that allowed attachment of other bacterial species (Sreekrishnan et al., 1991 Zellner et al., 1991 Yongming et al., 1993). This is contrary to findings in an acetate-propionate-butyrate degrading reactor, in which facultative anaerobes were found to be the initial colonizers (Lauwers et al., 1990). 

Figure 2.5. An example of a Spodosol the Kalkaska soil series, the state soil of Michigan (single-grain structure in horizon C means that all the sand particles act independently of each other) [2].

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