Inverse solubility

Precipitation or scaling fouling precipitation on hot surfaces or due to inverse solubility. 

Mineral scales typically result from the effects of localized concentration of salts within the watersides of a boiler and the inverse solubility of many such salts at elevated temperatures. Scales often are hard, dense, and difficult to remove. They can be either crystalline or amorphous (lacking any characteristic crystalline shape). 

The design of an evaporation unit requires the practical application of data on heat transfer to boiling liquids, together with a realisation of what happens to the liquid during concentration. In addition to the three main features outlined above, liquors which have an inverse solubility curve and which are therefore likely to deposit scale on the heating surface merit special attention. 

Aryltellurium trichlorides are highly soluble in methanol and ethanol but less soluble in benzene. Diaryltellurium dichlorides exhibit inverse solubilities, being more soluble in benzene than in methanol or ethanol. These properties allow an easy separation of diaryl tellurides from diaryl ditellurides (frequently formed as by-products in the preparation of tellurides) the mixture is treated with SOjClj and the obtained mixed di- and trichlorides are separated by the appropriate solvents, and reduced back into the pure tellurides and ditellurides. 

Solubility. PPO polyols with a molecular weight below 700 are water soluble. The triol is slightly more water soluble than the diol. The solubility in water decreases with increasing temperature. This inverse solubility causes a cloud point which is important in characterizing copolymers of propylene oxide and ethylene oxide. 

Hexamine is also known as hexamethylenetetramine, aminoform, crystamine, methenamine or formin. It was first prepared in 1859 by Butlerov of Russia. It is a white, crystalline powder with a slight amine odor. It is soluble in water, alcohol, and chloroform, but it is insoluble in ether. However the aqueous solutions exhibit inverse solubility, i.e., less hexamine dissolves as the temperature increases. The hydrate, (CHi N O can be crystallized from the aqueous solution at temperatures below 14°C. Some additional properties are listed in Table 17.1. 

As can be noted, the correlation between the calculated and actual results, using a very simple curve fitting technique, yields good fits concerning molar volume, which is examined in terms of density, and in cloud point data, examined in terms of tai jerature. For those not familiar with the cloud point phenomena, nonionic surfactants exhibit inverse solubility characteristics per taiperature increases, i.e., as the... 

Curve 3 shows that as the temperature increases, the solubility decreases. This is inverse solubility. Yield is obtained by evaporating the solvent or salting out. Heat exchangers must be designed using lower temperature increases and lower ATs between the heating media and the slurry. Sodium sulfate and sodium carbonate monohydrate exhibit this type of solubility. 

Properties Crystalline solid. Homopolymers and copolymers prepared with this material show inverse solubility in water. 

In contrast, inverse solubility salts are less soluble as the temperature of the solution is raised. Examples of inverse solubility salts are CaCOs and CaS04. It will be readily appreciated that water containing inverse solubility salts used for cooling purposes is likely to cause fouling problems because as heat is abstracted by the water, its temperature will rise, and if it is saturated with inverse solubility salts, precipitation of the salts will occur. In Fig. 6, the sequence of events that occurs when an inverse solubility salt is heated is illustrated. The curve represents the solubility variation with temperature. The solution of the salt at point A is not saturated. As the solution represented by A is heated it will eventually reach the solubility curve at... 

Fig. 6 Cooling an inverse solubility salt solution. (From RefPf)...

The conventional chemicals used for the prevention or limitation of scale formation (the deposition of inverse solubility salts) include threshold agents, crystal modifiers, dispersants, and surfactants. 

The question remains, however, of whether the solution is in fact infinitely dilute at a solute concentration of xi. Only if this is true is it valid to assume that yi = y - Literature values of solubility data for several compounds in water were used to obtain parameters for the UNIQUAC and NRTL excess Gibbs energy expressions, and y values for these compounds were calculated. The calculated values are compared with inverse solubility data in Table I. The inverse solubility predicts lower values of y in all cases. However, the difference becomes smaller as the solubility decreases, and for compounds with solubility less than 0,5% the difference is less than 10%. It has been shown that these excess Gibbs energy expressions, while very useful, are not the exact representation of the composition dependence of activity coefficient all expressions have difficulty in representing liquid-liquid equilibria (43-44). Thus, extrapolating these expressions to infinite dilution may be in error. It is therefore inconclusive as to the correctness of using the inverse solubility to calculate... 

Table I. Comparison of Inverse Solubility to y Calculated by the UNIQUAC and NRTL Expressions with Parameters Found from Mutual Solubility Data...

Crystallization. When salts having an inverse solubility characteristic precipitate on a heat transfer surface hotter than the flowing fluid. 

All polyalkylene glycols exhibit inverse solubility in water, i.e. water solubility decreases as temperature increases. This is explained by the loss of hydrogen bonding at elevated temperatures. The temperature of polymer/water separation is usually referred to as the cloud point and is higher for copolymers with larger proportions of ethylene oxide. 

There is a small demand within the soluble oil sector for fully synthetic true solutions , particularly for light-duty and grinding applications. These solutions may be formulated using water-soluble PAGs as base stock where their inverse solubility characteristic is advantageous in coating metal surfaces with a film of polymer to give lubrication and reduced tool wear. Biostability, low toxicity and little or no skin irritancy are additional benefits. 

The foaming properties of the nonionic surfactants depend upon the temperature because of their inverse solubility temperature relationship. Above the cloud point they are nonfoamers and some nonionic surfactants may even function as defoamers above their cloud point temperature. Therefore, the nonionic surfactant selected for rinse aid formulations must have a cloud point below the temperature of the rinse water. 

One of the main features of nonionic water-soluble cellulose derivatives is that they exhibit, like some other polyethers, an inverse solubility-temperature behavior, i.e. there is phase separation on heating above the so-called lower critical solution temperature (LCST). The temperature at which a polymer-rich phase separates is normally referred to as the cloud point (CP). For ideal solutions, this temperature corresponds to the theta-temperature. Actually, for some derivatives, the cloud point may be preceded, if the concentration is not too low, by a sol-gel transformation with an increase in viscosity and possibly formation of liquid crystals (see Sect. 3.5). As it will be seen later, this reversible thermotropic behavior may be detrimental to the performance of the derivatives or can be advantageneously utilized to develop applications. 

Two forms of solubility are possible. Some salts have greater solubilities as the temperature is raised these s ts are called normal solubility salts and examples include NaCl and NaNOy Other salts usually termed inverse solubility salts have lower solubilities as the temperature is rmsed. Examples of inverse solubility salts include CaCO and CaSO. Table 8.4 gives a more extensive list of inverse solubility salts. The relevance of these salts to the components present in river water listed in Table 8.1 is clear. The use of these waters for cooling purposes is very likely therefore, to cause fouling or scaling problems due to the presence of the inverse solubility salts. 

These different characteristics of temperature and solubility can have a pronounced effect on scaling depending on whether or not the saturated solution is being heated or cooled. Inverse solubility salts are likely to form a deposit if the saturated solution is being heated the reverse is true for normal solubility salts. Problems are likely only to be encountered for sparingly soluble salts where relatively small changes of temperature have a significant effect on solubility. 

The sequence of events for an inverse solubility salt solution are illustrated on Fig. 8.6 [Bott 1990]. A solution at A is undersaturated as it is heated it reaches the... 

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