Splash zone

A generic multipurpose fluidized bed is illustrated in Figure 2 (1). The soHds are contained in a vessel and gas is introduced into the system via a distributor, which is typically a drilled plate at the bottom of the vessel. A plenum chamber is provided below the distributor plate. The height of the soHds level above the distributor is called the bed height, and the vertical space above the bed height is called the freeboard. A splash zone may exist as a transition between the bed and freeboard. Cyclones, located either in the freeboard or external to the vessel, are used to remove soHds from the gas stream. Diplegs can return entrained soHds directly to the bed. 

Corrosion susceptibility in aqueous media is assessed on the basis of the rating numbers [3, 14], which are different from those of soils. An increased likelihood of corrosion is in general found only in the splash zone. Particularly severe local corrosion can occur in tidal regions, due to the intensive cathodic action of rust components [23, 24]. Since cathodic protection cannot be effective in such areas, the only possibility for corrosion protection measures in the splash zone is increased thickness of protective coatings (see Chapter 16). In contrast to their behavior in soils, horizontal cells have practically no significance. 

In the tidal zone and the spray zone (known as the splash zone), cathodic protection is generally not very effective. Here thick coatings or sheathing with corrosion-resistance materials (e.g., based on NiCu) are necessary to prevent corrosion attack [4]. The coatings are severely mechanically stressed and must be so formed that repair is possible even under spray conditions. Their stability against cathodic polarization (see Section 17.2), marine growths, UV rays and seawater must be ensured [4,5]. 

The difficulties of such operations on the research platform Nordsee are described in Ref. 9. The Murchison platform was provided with a combination of impressed current protection and galvanic anodes because there was a limit to the load to be transported [12]. The anodes for platforms are installed and provided with cables at the yard. They are installed with redundancy and excess capacity so that no repairs are necessary if there is a breakdown. The lower part of the platform up to the splash zone is usually placed in position in the designated location at least 1 year before the erection of the deck structure so that impressed current protection cannot initially be put in operation. This requires cathodic protection with galvanic anodes for this period. This also means that the impressed current protection is more expensive than the galvanic anodes. 

Installation Coating Splash zone (m ) Current density (mA m ) Type of anode No. of anodes Anode weight (t) Rectifier output (A) Life (years)... 

Residual stresses occur from welding and other fabrication techniques even at very low stress values. Unfortunately, stress relief of equipment is not usually a reliable or practical solution. Careful design of equipment can eliminate crevices or splash zones in which chlorides can concentrate. The use of high-nickel stainless steel alloy 825 (40% nickel, 21% chromium, 3% molybdenum and 2% copper) or the ferritic/austenitic steels would solve this problem. 

The presence of shell fouling affects the corrosion of steel structures in the intertidal zone where it has been found that the rust formed consists of irregular layers or iron oxides and lime, the latter accounting for up to 15% by weight of the corrosion product". The corrosion rate of mild steel in UK waters for the full immersion and intertidal zone is typically 0.08 mm/y compared with 0.1 to 0.25 mm/y in the splash zone according to the strength of wave action. Above the splash zone corrosion diminishes rapidly to 0.05-0.1 mm/y". 

From a consideration of Table 3.10, a copper-phosphorus steel might be chosen for its resistance to corrosion in the critical tidal and splash zone. However, the variation of corrosion resistance is much greater than the difference between various alloy steels so it is improbable that low-alloy steels will corrode more slowly than mild steel in most practical environments. This conclusion is supported by Forgeson etal. who concluded from extensive tests in fresh and salt waters of the Panama Canal Zone that Proprietary low-alloy steels were not in general more resistant to underwater corrosion than the mild unalloyed carbon steel . ... 

The so-called splash zone , the head space in a mixing vessel above the bulk phase of product, may be irrigated with condensation water, washing less persistent biocides away and allowing microbial growth to occur in this area. 

Condensation > irrigates splash zone - biocide washed away... 

The larger particles were thrown high in the splash zone higher than predicted by a ballistic trajectory using the bubble rise velocity as the initial velocity and neglecting any air drag. Later observations of this model showed that, when bubbles erupt at the surface, the accompanying... 

Experimental Studies on the Contribution of the Splash Zones in Countercurrent Cooling Water Towers... 

A typical example of a circulating fluidized-bed reactor model has been presented by Schocnfeldcr et al. [112, 116]. Its structure shown in Fig. 21 is based on the definition of four axial zones. Above the bottom zone the splash zone is located. It yields a mixing condition to link the bottom zone with the upper part of the reactor. Due to the internal backmixing this upper section is referred to as the recirculation zone. At the riser outlet, an exit zone on top of the recirculation zone yields a second boundary condition. Here, complete mixing is assumed. 

The splash zone above the free bubbling zone consists of entrained droplets. We may model this zone as being made up of droplets of uniform size rising in plug flow through the splash zone. The model parameters are... 

The contribution to the total mass transfer of the splash zone is generally negligibly small and is neglected in the working model for mass transfer calculations pictured in Figure 12.4. 

It is interesting to note a further analogy between tray hydrodynamics and gas-solid fluidized beds the splash zone is akin to the freeboard zone in the latter case. 

The splash zone at the top of the dispersion has the same characteristics as the one described above for the free bubbling zone its contribution to mass transfer can usually be neglected. 

Another potential pest of timbers in maritime situations is the wood-boring weevil Pselactus spadix that can excavate galleries in the inter-tidal and splash zones of wharf timbers. The larvae are believed to be the primary tunnellers,... 

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