Inverse solubility salts

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. 

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... 

Suitor et al [1976] reviewed the temperature effects for inverse solubility salts. An important factor, sometimes overlooked, is that as the deposition process continues the deposit temperature rises at constant heat flux. Under these higher temperature conations, some process of additional crystallisation and reorientation are likely to occur [Taborek 1972]. The strength of the deposit, and the ease with which it can be removed, may be affected by these additional processes. 

As already described it is posible for supersaturation to occur if a normal solubility salt is cooled or an inverse solubility salt is heated. Apart from temperature there are other conditions that could lead to supersaturation and hence crystallisation, and may include ... 

Precipitation Fouling the crystallization from solution of dissolved substances onto the heat transfer surface, sometimes called scaling. Normal solubility salts precipitate on subcooled surfaces, while the more troublesome inverse solubility salts precipitate on superheated surfaces. 

The former case is usually associated with the use of evaporators for crystallization objectives. Crystal growth on the heat transfer surface competes with the process of crystal growth on the greater deposition area of the suspended crystals. Supersaturation with respect to the heat transfer surface which has e higher temperature than the bulk solution is lower for normal solubility salts and higher for inverse solubility salts. 

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 largest solubility isotope effects are found for sparingly soluble salts. For example, lead chloride and potassium bichromate are 36% and 33.5% more soluble in H20 than D20 at 298.15 and 278.15 K, respectively. For the more soluble salts, NaCl and KC1, the values are 6.4% and 9.0%. Interestingly LiF and LiCl.aq have inverse effects of 13% and 2%, respectively. Recall that lithium salts are commonly designated as structure makers . Almost all other electrolytes are structure breakers . 

The concentration of lactose is inversely related to the concentration of soluble salts expressed as osmolarity. This results from the requirement that milk be isotonic with blood. 

Reciprocal action of two soluble salts within a solution.—The phenomena of etherification, studied by Berthelot and Pdan de Saint-Giles, are not the only ones in which may be observed a state of chemical equilibrium, the common limit of two reactions which are the inverse of each other. Berthollet had predicted that such a state of equilibrium should be produced in a solution in which two soluble salts may, by double decomposition, produce two other soluble salts. Malaguti has verified the truth of Berthollet s prediction in the following way ... 

Idea of chemical equilibrium. It differs from the idea of medianical equilibrium, page 58.-46. The chemical equilibrium may be the common limit of two oppositely directed reactions. Phenomena of etherification, 58.— 47. Reciprocal actions of two soluble salts in the midst of a solution, 55.—48. Many chemical systems seem incapable of possessing a state of equilibrium which is the common limit of two reciprocally inverse reactions, 66.— 49. Grove s experiment. Water is decomposable by heat, 57.—50. Direct demonstration of the dissociation of water, 57.—51. Dissociation of carbonic acid gas, 59.-52. These decompositions are not complete but limited at the temperatures at which they are produced, the inverse reaction also takes place, 59.—53. Example of a state of equilibrium which is the common limit of two reactions the Inverse of each other. Action of water vapor on iron and the inverse action, 61.—54. Changes of physical state give rise to equilibrium conditions of which each is the common limit of two modifications the inverse of each other. 

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. 

Figure 11 The preparation of nanoparticles by the inverse micelle process in which a chemical reaction between microemulsion or inverse micelles after collision and perhaps fusion converts the soluble salt into an insoluble metal or metal oxide as shown. Source From Ref. 75.

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

Heating above the solubility limit for inverse solubility (increasing solubility with increasing temperature, such as calcium and magnesium salts). The precipitation of salt occurs with heating the solution. 

Some typical applications of mechanical vapor recompression units are evaporation of sea water to give distilled water, evaporation of kraft black liquor in the paper industry (L2), evaporation of heat-sensitive materials such as fruit juices, and crystallizing of salts having inverse solubility curves where the solubility decreases with increasing temperature (K2, M3). 

A flash evaporator system having no heating surfaces has been developed for separating salts with normal solubility from salts having inverse solubility. Steam is injected directly into the feed slurry to dissolve the normal-solubility salt by increasing the temperature and dilution of the slurry. The other salt remains in suspension and is separated. The hot dilute solution is then flashed to a lower temperature where the normal-solubility salt crystallizes and is separated. The brine stream is then mixed with more mixed salts and recycled through the system. This system can be operated as a multiple effect by flashing down to... 

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