Temperature inversion deposition

Components in which water temperature changes abruptly with distance, such as heat exchangers, tend to accumulate precipitates. Heater surfaces also accumulate precipitates if the dissolved species have inverse temperature solubilities. Systems in which pH excursions are frequent may accumulate deposits due to precipitation processes. Plenum regions, such as heat exchanger headboxes, tend to collect deposits. 

Calcium carbonate has normal pH and inverse temperature solubilities. Hence, such deposits readily form as pH and water temperature rise. Copper carbonate can form beneath deposit accumulations, producing a friable bluish-white corrosion product (Fig. 4.17). Beneath the carbonate, sparkling, ruby-red cuprous oxide crystals will often be found on copper alloys (Fig. 4.18). The cuprous oxide is friable, as these crystals are small and do not readily cling to one another or other surfaces (Fig. 4.19). If chloride concentrations are high, a white copper chloride corrosion product may be present beneath the cuprous oxide layer. However, experience shows that copper chloride accumulation is usually slight relative to other corrosion product masses in most natural waters. 

The precipitation of anhydrite (anhydrous calcium sulfate, CaS04) may also occur. Under ambient temperatures, anhydrite is much more soluble than calcium carbonate, but because calcium sulfate, in common with other calcium salts such as calcium phosphate (also known as tricalcium phosphate [Ca3(P04)2]), has an inverse-temperature solubility, it deposits more rapidly on the hottest heat transfer surfaces. 

Most salts absorb heat when they go into solution, and their solubility increases with a rise in temperature however, calcium carbonate (CaC03), in common with several other anhydrous salts such as calcium sulfate (CaS04) and calcium phosphate [Ca3(P04)2], has an inverse temperature solubility and thus readily precipitates to form deposits in hot water areas (FW tanks, FW lines, and boiler heat exchange surfaces). 

Several common salts have an inverse temperature solubility and readily precipitate to form deposits on hot boiler surfaces and other heat exchange areas. These include ... 

Hirn demonstrated that saturated steam when expand adiabatically in a cylindrical copper vessel with plate-glass ends deposits droplets of liquid, visible as a fog. Cazin connected the cylinder with another containing a piston, so that the vapour could be adiabatically compressed as well as expanded. Steam and carbon disulphide vapours (or"<0), condensed on expansion, but ether vapour (or">0) on compression. The inversion temperatures for benzene and cUoroform were about 120° and 127°C. respectively, agreeing with those calculated from Regnaulfs results, for CS2 790°, ether — 113°, CHCl3 123-5°, benzene 100° C. 

Galvanic corrosion occurs because of (i) nonmetalUc conductors and corrosion products, (ii) metallic coatings and sacrificial anodes, (iii) polarity inversion, (iv) deposition corrosion, (v) hydrogen cracking or damage, (vi) high temperature. 

The Arrhenius plot of deposition rate as a function of inverse temperature is shown in Fig. 10. The deposition rate is found to vary from 1.9 nm/min to 49 nm/min depending on the deposition temperature and carrier gas used. It linearly increases up to 1073 k for both silicon nitride and silicon carbonitride, indicating that, in this... 

Fig. 10. Arrhenius plot of deposition rate versus inverse temperature,...

Fig. 43. Plots of In(o-) vs inverse temperature for PANI in 1-M HCIO4 films deposited by potentiodynamic cycling (1), potentiostatic deposition (2), square wave deposition (3), nitrogen purged [445].

Figure 6-14. Average domain size vs. inverse deposition temperature Tor different film thicknesses. Error bars represent the mean absolute error and straight lines the best lit for each film thickness. Doited line is the locus of the transition from grains to lamellae. Data for 50-nm films are estimated from the correlation length of the topography fluctuations. Adapted from Ref. [501.

Pressure is similar to temperature as a rate limiting factor since the diffusibility of a gas is inversely related to its pressure. For instance, loweringthe pressure 760 Torr(l atm)to 1 Torr increases the gas-phase transfer of reactants to the deposition surface and the... 

In both cases, the Au nanoparticles behave as molecular crystals in respect that they can be dissolved, precipitated, and redispersed in solvents without change in properties. The first method is based on a reduction process carried out in an inverse micelle system. The second synthetic route involves vaporization of a metal under vacuum and co-deposition of the atoms with the vapors of a solvent on the walls of a reactor cooled to liquid nitrogen temperature (77 K). Nucleation and growth of the nanoparticles take place during the warm-up stage. This procedure is known as the solvated metal atom dispersion (SMAD) method. 

Fig. 30. Hydrogen spin lattice relaxation time T, in a-Si H against temperature for flake samples removed from their substrate (solid line) and for a-Si H on quartz substrates two weeks after deposition (triangles). The circle data points are for the quartz substrate samples ten months after deposition. The magnitude of the 40 K minimum of T, is inversely portional to the number of H2 molecules contributing to the relaxation process (Van-derheiden et al., 1987). 

Clearly, the concentrations of pollutants in ambient air, and hence their impacts, are determined not only by their rates of emissions but also by the nature and efficiencies of their chemical and physical sinks, e.g., chemical transformations, as well as wet and dry deposition to the earth s surface. To a large extent, these competing processes are affected not only by direct dispersion and transport but also by such meteorological factors as temperature, sunlight intensity, and the presence of temperature inversions as well as clouds and fogs. 

It was also noted that the optimal pH was temperature dependent At low deposition temperature it was 9, while at high temperatures it was 10. This follows from the inverse dependence of rate on pH. The optimal pH is a balance between slow-enough formation of PbSe (to prevent precipitation in solution) but not too slow to prevent formation of a film in a reasonable time. At higher temperatures the rate is faster, and therefore the optimal pH should be lower. 

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