Wetting systems, examples

Finally we will Illustrate the various transition steps between a solid surface under pristine conditions and the completely Immersed state, (see fig. 5.16). Panels (a), (c) —> (d) (e) (g) refer to a wetting system. Examples are water on various... 

Average temperatures (40-70°C) in combination with high-shear dispersion equipment, of which three-roll mills are a good example, appear to afford the best results in terms of pigment dispersion in wetting systems, such as offset varnish. Fig. 33 shows a curve in which the degree of dispersion reaches a distinct maximum at one particular temperature. 

The operation of molecular devices in wet systems can yield performances unobtainable in dry systems. For example, molecular devices in wet systems can provide characteristic electron transfer control. While wet systems have a disadvantage in performance speed because of the slow mobility of ions, they have a notable advantage in fine and precise control of the direction and kinetics of electron transfer, even at room temperature. This characteristic can lead to a low noise level, because electron transfer is governed by the absolute electrochemical potentials of a series of molecules coexisting in the system. 

You can measure the contact angle by determining the spreading of a small drop of known volume, for example under a microscope. Figure C2-9 shows the diameter of the drop as you see it from above. For a non-wetting system, you see the diameter of the sphere, not that of the contact disk. Results from this part of the graph are not accurate. 

It is interesting to compare currents in the wet and dry systems. For an example, one of Mead s Figures (our Figure 16) shows 0.1 A at room temperature for 10 cm at 36 V and 870 A, i.e., 10 A/cm at 5 X 10 V/cm, with tantalum negative. According to Figure 18 the asymmetry was small. In the wet system with tantalum positive one would observe about 10 A/cm at this field and this current would be nearly all ionic. 

The condition that the electrochemical processes occur largely in the region of the meniscus is only met if the thin film of electrolyte is absent or if the thickness is very small. The simple-pore model [64] is an example of the first case. The meniscus is assumed to form at the intersection of micropores with macropores. While the micropores are filled with electrolyte up to the intersection, the macropores are filled with gas. The meniscus may be treated as flat in a first approximation. The walls of micropores are the seat of the electrode reaction. The simple-pore model was suggested [64] as applying to non-wetted systems like the Teflon-bonded platinum black electrodes. The limiting current due to the diffusion of species into a micropore was derived [64] as the steady-state solution of the two-dimensional diffusion equation in cylindrical coordinates. The summation of the currents of the individual pores leads to ... 

Clean sandstone tends to be water-wet, but many sandstone reservoir rocks are intermediate-wet. Carbonates tend to be more oil-wet than elastics. In gas-liquid systems, gas is always the non-wetting phase. Frequently water-wet, for example, are North-Sea sandstone reservoirs, whereas frequently oil- or mixed-wet are Middle East carbonate reservoirs. 

Emissions control systems play an important role at most coal-fired power plants. For example, PC-fired plants sited in the United States require some type of sulfur dioxide control system to meet the regulations set forth in the Clean Air Act Amendments of 1990, unless the boiler bums low sulfur coal or benefits from offsets from other highly controlled boilers within a given utiUty system. Flue-gas desulfurization (FGD) is most commonly accomphshed by the appHcation of either dry- or wet-limestone systems. Wet FGD systems, also referred to as wet scmbbers, are the most effective solution for large faciUties. Modem scmbbers can typically produce a saleable waUboard-quaUty gypsum as a by-product of the SO2 control process (see SULFURREMOVAL AND RECOVERY). 

These relationships predict the binding Hquid content for wet agglomeration with an accuracy of only ca 30%. The Hquid content required to agglomerate a particular feed material depends, for example, on the interfacial properties of the system (45). Typical values of moisture content required for hailing a variety of materials are listed in Table 2. Very accurate information on the optimum Hquid content to agglomerate a particular feed material must be obtained from experimental tests. 

Because powdered activated carbon is generally used in relatively small quantities, the spent carbon has often been disposed of in landfills. However, landfill disposal is becoming more restrictive environmentally and more costiy. Thus large consumers of powdered carbon find that regeneration is an attractive alternative. Examples of regeneration systems for powdered activated carbon include the Zimpro/Passavant wet air oxidation process (46), the multihearth furnace as used in the DuPont PACT process (47,48), and the Shirco infrared furnace (49,50). 

A variety of specialized idlers are available. Examples are plastic disk catenary idlers for handling wet corrosive materials two roU idlers, where the roUs are oriented in a vee for lighter duty conveying system and suspended idler supports for severe service. In this last type, three to five idler roUs are linked end-to-end and suspended from conveyor frame stringers to form a catemary that cradles the belt. 

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