Inverted solubility

Calcium sulfate, which exists in sea water in ionic form, has a reverse or inverted solubility curve above about 37° C.—that is, solubility decreases with increasing temperature. 

Scale Prevention. The scale normally formed on heat transfer surfaces of sea water evaporators consists of calcium carbonate, magnesium hydroxide, and/or calcium sulfate. The first two form as a result of the breakdown of bicarbonate in sea water, which is initially saturated with calcium carbonate. Calcium sulfate scale forms purely as a result of its inverted solubility curve. Sea water is not saturated with calcium sulfate and an economically reasonable amount of fresh water can be recovered from sea water without exceeding saturation with calcium sulfate. However, at the start of this investigation, the solubility of calcium sulfate in sea water was not accurately enough known to tell whether 30, 50, or 80% of the water content could be removed at various temperatures without encountering calcium sulfate scale. 

Scale formation occurs to some extent in all types of evaporators, but it is of particular importance when the feed mixture contains a dissolved material that has an inverted solubility. The expression inverted solubility means the solubility decreases as the temperature of the solution is increased. For a material of this type, the solubility is least near the heat-transfer surface where the temperature is the greatest. Thus, any solid crystallizing out of the solution does so near the heat-transfer surface and is quite likely to form a scale on this surface. The most common scale-forming substances are calcium sulfate, cal-... 

One benefit of this design is that with materials of flat or inverted solubility, the use of recompression complements the need to maintain low ATs to prevent fouling of the heat-transfer surface. 

Since all scale-forming constituents of saline waters have inverted solubility-temperature relationships, the scale problem can be alleviated, if not wholly solved, by evaporation at high vacuum and hence low temperature. Some small single-stage units for household and other uses boil the solution at a temperature as low... 

When the heat exchanger is used, it normally is one pass or two pass, and is designed for relatively low temperature rises of the solution pumped through the exchanger. This limits the supersaturation when heating materials of inverted solubility. In most applications, the steam-liquid AT is also limited so as to prevent mass boiling of the solution pumped through the tubes or vaporization at the tube wall. 

For most evaporative units, the steam flow is directly proportional to the production rate and to the temperature rise in the recirculation system and the AT" in the heat exchanger. Both of these values have an important influence on the scaling rate in cases where materials of inverted solubility are being handled or where there are scale-forming components within the system. In systems where this value is critical, the AT can be monitored and used to reset the steam rate. An interlock should be provided that will cut off the steam flow to the system if the circulating pump or propeller circulator in a draft tube baffle unit is off due to motor overload or power failure. 

When a solute dissolves in a solvent without reaction, heat is usually absorbed from the surrounding medium (still commonly referred to as the heat of solution), i.e. if the dissolution occurs adiabatically the solution temperature falls. When a solute crystallizes out of its solution, heat is usually liberated (still commonly referred to as the heat of crystallization) and the solution temperature rises. The reverse cases, viz. heat evolution on dissolution and heat absorption on crystallization, may be encountered with solutes that exhibit an inverted solubility characteristic, e.g. anhydrous sodium sulphate in water. 

Not all solubility curves are smooth, as can be seen in Figure 3.1b. A discontinuity in the solubility curve denotes a phase change. For example, the solid phase deposited from an aqueous solution of sodium sulphate below 32.4 °C will consist of the decahydrate, whereas the solid deposited above this temperature will consist of the anhydrous salt. The solubility of anhydrous sodium sulphate decreases with an increase in temperature. This negative solubility effect, or inverted solubility as it is sometimes called, is also exhibited by substances such as calcium sulphate (gypsum), calcium, barium and strontium acetates, calcium hydroxide, etc. These substances can cause trouble in certain types of crystallizer by causing a deposition of scale on heat-transfer surfaces. 

The general trend of a solubility curve can be predicted from Le Chatelier s Principle which, for the present purpose, can be stated when a system in equilibrium is subjected to a change in temperature or pressure, the system will adjust itself to a new equilibrium state in order to relieve the effect of the change. Most solutes dissolve in their near-saturated solutions with an absorption of heat (endothermic heat of solution) and an increase in temperature results in an increase in the solubility. An inverted solubility effect occurs when the solute dissolves in its near-saturated solution with an evolution of heat (exothermic heat of solution). 

Many aqueous and organic systems exhibit eutectic and incongruent points. Several cases are known of an inverted solubility effect after the transition point (see Figure 3.1b) the systems Na2S04-H20 and Na2C03-H20 are particularly well-known examples of this behaviour. 

Precipitation fouling giving scale or sludge/ soluble spedes present in feed/tem-perature high for invertly soluble/temperature too low for incrustation or crystal formation. 

The same research group designed a soluble catalyst 102 with the main aim of avoiding the problems associated with the heterogeneous systems, and related to the common supported catalysts [207]. The main advantage of this system is the inverted solubility pattern that this catalyst exhibits, since it is soluble in non-polar solvents and insoluble in polar media (Fig. 9). This feature simplified the recovery (up to 99 %) and re-use of the catalyst at least five times without loss of activity, improving the results obtained with catalyst 100 (for the preparation of other immobilized catalysts easily recoverable, see [208]). 

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