Chemical supersaturation

In this equation the concentrations are not the equilibrium values but the actual values in the reactor. This definition corresponds to that of the supersaturation used with physical condensation. Physical supersaturation is a special case of chemical supersaturation as defined above if the condensation reaction is A g) - C(5), the definition given above reduces to S = [A]reactor/[ ]equii.> which defines physical supersaturation. There is saturation when S = 1, and p(A) can be used instead of [A]. 

Define chemical supersaturation. What is its temperature dependence Explain the function of impurities for Ti02-formation shown in Figure 6.17. 

Expressions of supersaturation can then be formulated as follows (/) the difference between the chemical potential of the system and the chemical potential of at saturation, ji — ji where the chemical potential is a function of both temperature and concentration (2) the difference between the solute concentration and the concentration at equiUbrium, c — c (J) the difference between the system temperature at equiUbrium and the actual temperature,... 

Any of these definitions of supersaturation can be used over a moderate range of system conditions. For example, a difference ia chemical potential A// = — fi is proportional to both c — c and T — T over a modest range of conditions. Of the five expressions given, however, the second is most... 

Carbon dioxide gas diluted with nitrogen is passed continuously across the surface of an agitated aqueous lime solution. Clouds of crystals first appear just beneath the gas-liquid interface, although soon disperse into the bulk liquid phase. This indicates that crystallization occurs predominantly at the gas-liquid interface due to the localized high supersaturation produced by the mass transfer limited chemical reaction. The transient mean size of crystals obtained as a function of agitation rate is shown in Figure 8.16. 

The reactor has been successfully used in the case of forced precipitation of copper and calcium oxalates (Jongen etal., 1996 Vacassy etal., 1998 Donnet etal., 1999), calcium carbonate (Vacassy etal., 1998) and mixed yttrium-barium oxalates (Jongen etal., 1999). This process is also well adapted for studying the effects of the mixing conditions on the chemical selectivity in precipitation (Donnet etal., 2000). When using forced precipitation, the mixing step is of key importance (Schenk etal., 2001), since it affects the initial supersaturation level and hence the nucleation kinetics. A typical micromixer is shown in Figure 8.35. 

Larson, M.A. and Klekar, S. A., 1973. In-situ measurement of supersaturation in crystallization from solution. Presented at American Institute of Chemical Engineers 66th Annual Meeting, Philadelphia, November. 

In view of the selective character of many colorimetric reactions, it is important to control the operational procedure so that the colour is specific for the component being determined. This may be achieved by isolating the substance by the ordinary methods of inorganic analysis double precipitation is frequently necessary to avoid errors due to occlusion and co-precipitation. Such methods of chemical separation may be tedious and lengthy and if minute quantities are under consideration, appreciable loss may occur owing to solubility, supersaturation, and peptisation effects. Use may be made of any of the following processes in order to render colour reactions specific and/or to separate the individual substances. 

Under these conditions, localized mineral supersaturation will lead to the precipitation of various scales and their ultimate deposition on the heat transfer surface without the possibility of scale resolubliza-tion. This is because surface washing by bulk BW (with or without the presence of water treatment chemicals) cannot take place due to the presence of the insulating steam blanket. 

Deposition commonly reflects a combination of physicochemical processes and localized effects. It may occur through fouling as a result of contamination by process materials, perhaps plus scaling from the supersaturation of dissolved salts, and coupled with some active under-deposit corrosion. As a consequence, deposits forming within a boiler are almost never single mineral scales but typically consist of a variable mix of scale and corrosion debris, chemical treatment residuals, process contaminants, and the like. 

NOTE Control over some problems, such as the supersaturation of dissolved solids, short-term and long-term overheating, and boiler priming (surging), relies as much on good operation as on the use of appropriate chemical treatments. 

If we could prevent the mixture from separating into two phases at temperatures below Tc, we would expect the point of inflection to develop into curves similar to those shown in Figure 8.17 as the dotted line for /2, with a maximum and minimum in the fugacity curve. This behavior would require that the fugacity of a component decreases with increasing mole fraction. In reality, this does not happen, except for the possibility of a small amount of supersaturation that may persist briefly. Instead, the mixture separates into two phases. These phases are in equilibrium so that the chemical potential and vapor fugacity of each component is the same in both phases, That is, if we represent the phase equilibrium as... 

The manner in which a film is formed on a surface by CVD is still a matter of controversy and several theories have been advanced to describe the phenomena. ] A thermodynamic theory proposes that a solid nucleus is formed from supersaturated vapor as a result of the difference between the surface free energy and the bulk free energy of the nucleus. Another and newer theory is based on atomistic nucle-ation and combines chemical bonding of solid surfaces and statistical mechanics. These theories are certainly valuable in themselves but considered outside the scope of this book. 

In metal deposition, the primary products form adsorbates on the electrode surface rather than a supersaturated solution. Their excess chemical potential is directly related to polarization and given by nFAE. The total excess surface energy = 2 S,o,. Otherwise, all the results described above remain valid. 

Supersaturation can also be achieved by adding a liquid that is miscible with the solvent and decreases the solubility of the solute in the mixed solvent. This is called precipitation. In fine chemicals manufacture, the solid is usually dissolved in an organic solvent and water is used as the desalting agent. Precipitation also occurs when a solid product, which is insoluble in the reaction mixture, is formed by chemical reaction. For instance, a phenolic product can be purified by three possible routes ... 

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