Dry wetting

Cooling towers are broadly classified on the basis of the type of draft natural draft (natural convection), mechanical draft (forced convection) and mechanical and natural. Further distinction is made based on (1) the type of flow i.e. - crossflow, counterflow, cocurrent flow (2) the type of heat dissipation-wet (evaporative cooling), dry, wet-dry and (3) the type of application-industrial or power plant. Each of the major types of cooling towers has a distinct configuration. The major designs are summarized in Figures 1 through 8 and a brief description of each follows. 

The plant layout design and whether the processes involve dry, wet or contaminated environments with possibly elevated temperatures will affect the principles adopted... 

Table 1. Comparison between water uptakes, dry, wet and theoretical glass transition temperatures of TGDDM-DDS samples...

This basic mbber friction process is present on all surfaces, dry, wet or icy, being modified only by the external conditions. On wet surfaces this is primarily water lubrication which itself is influenced by the water depth, roughness of the road surface, and the state of the tire tread pattern. The low friction on ice near its melting point is mainly due to the properties of the ice. 

A violent explosion followed the use of magnesium perchlorate to dry wet fluo-robutane. The latter was presumed to have hydrolysed to give hydrogen fluoride which had liberated perchloric acid, explosively unstable when anhydrous. (This explanation seems unlikely in view of the large disparity between dissociation constants of the two acids). Magnesium perchlorate is unsuitable for drying acidic or flammable materials calcium sulfate would be suitable. 

A combination of SIPS with the stabilising and synthesis-favouring properties of clay minerals was studied by Rode et al. (1999) in experiments involving dry/wet cycles. The simultaneous use of both SIPS and clay minerals as catalytically active surfaces led to peptides up to and including the hexamer (Gly)6. The question as to whether this technique fulfils prebiotic conditions can (within certain limitations) be answered positively, since periodic evaporation phases in limited areas (lagoons, ponds) are conceivable. The container material could have consisted of clay minerals. Further progress in the area of peptide synthesis under conditions which could have been present on the primeval Earth can be expected. 

CoAF catalysts (Fig. 1) exhibited good NO reduction activity in the range 350-600°C, yielding N2, C02 and H20 as the main products, with a maximum NOx dry conversion of 20% and about 40%, respectively, on Co2.5AF at 550°C and Co4.2AF at 500°C. Under dry-wet cycles (Ciambelli et al., 2007) CoAF catalysts showed a progressive decrease of catalytic activity, which was not recovered in the subsequent dry tests. 

In dehydrated CoAF the migration of Co2+ ions from Co2 to Co2a sites on heating (Fig. lb) allows the occupancy of the most active a-sites where Co2+coordinates four framework oxygens. The irreversible and progressive decline of catalytic activity during dry-wet-dry cycles is likely to be due to cation hydration and movement from those sites, thus, decreasing their occupancies. 

Figure 2. (a) NOx (full symbols) and CH4 (void symbols) conversions on Ag3.7Co2.6AF with dry feed and (b) NOx conversion on Ag2.7Co2.8AF in dry-wet cycles... 

However, a loss in catalytic activity is generated during dry-wet cycles, corresponding to a decrease in Co2 site occupancy and in a relevant occupation in Ag2. In this position Ag+, half-way between Co2 and Co2a, hinders or interferes with Co migration to the Co2a active sites. 

On the basis of the very low Ag content, from the Co4.6AgO.2AF Rietveld refinement (Fig.5) Col, Co2 and also Ag2 were attributed to Co2+, whereas Ag5 must be partially occupied by Co2+ (for simplicity, site Ag2 will be indicated in Fig 5 and called Co4 from now on). As expected, no metallic Ag° was detected. After catalytic testing a fairly similar distribution of the 3.2 extraframework cations per unit cell was found. The most remarkable modification induced by the dry-wet SCR cycle was the migration of Co atoms from the Co2 to the Co4 site, an effect of particular interest as Co4 is, as stated earlier, nearer than Co2 to the Co2a position, i.e. the most active oc-site. 

This increase in the relative Co population for the Co2a position explains the increase in the occupancy of Co4 lost from the Co2 site after a dry-wet SCR cycle. 

We suggest that the absence of Ag+ in site Ag2 for Co4.6Ag0.2AF, (= Co4) which could hinder Co migration to the Co2a site and that of Ag°, together with the increased Co4 population lost from the Co2 site after a dry-wet SCR could be responsible for preserving SCR catalytic activity and selectivity for low Ag content bimetallic samples. 

Denef K, Six J, Bossuyt H, Frey SD, Elliott ET, Merckx R, Paustian K (2001) Influence of dry-wet cycles on the interrelationship between aggregate, particulate organic matter, and microbial community dynamics. Soil Biol Biochem 33 1599-1611... 

To confirm this idea two examples are given in Figures 18(a) and (b) ecosystem-dependent depositions of lead in South Norway and in Central Spain in 2002. Depositions are split in wet and dry. Wet deposition fluxes are assumed to be the same for different types of ecosystems. Annual precipitation amounts in these regions are about 1,400 (Norway) and 510 (Spain) mm. In the Norwegian region dry deposition to forests is higher than that to arable lands. However, due to the large amount of precipitation, wet deposition prevails and total deposition (sum of wet and dry) does not differ much between forests and arable lands. 

HS-I diet and fecal samples were dry ashed and HS-II and -III samples were prepared by a combination dry-wet ashing procedure (Hill et al. unpublished). The approved method of American Association of Cereal Chemists (7) was used for determining neutral detergent fiber. 

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