Supersaturation Crystallization Processes | Methods of Supersaturation

Consider an equilibrium solution (solid solute + liquid solvents). If we disturb this equilibrium either by cooling the solution or evaporating a portion of the solvent then the quantity of solute will exceed the equilibrium concentration and the system will try to attain a new state of equilibrium by expelling excess solute present in it in the form of crystals. This process of forming crystals is called crystallization form solution and the concentration different driving force is supersaturation. 

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Supersaturation is the quantity of solute present in a solution (in which crystals are growing) compared with the quantity of solute that is in equilibrium with the solution. The supersaturation is expressed as a coefficient, given as,

S = (Parts solute/100 parts solvent) at prevailing condition/(Parts solute/100 parts solvent) at equilibrium > 1.0

Crystallization cannot take place/occur unless a solution is supersaturated. The number of crystals formed however depends upon the difference in saturation concentration since once the crystallization begins the extra solute held in the solution due to supersaturation also comes out of the solution in the form of crystals. 

>Types of Reactants | What are Reactants in Chemistry 

Methods of Supersaturation 

Unless a solution is supersaturated, neither nucleation nor crystal growth occurs. Thus for crystallization to occur, supersaturation can be generated by any one of the following methods:

(a) By cooling a concentrated, hot solution through indirect heat exchange. 

(b) By evaporating a part of the solvent/By evaporating a solution. 

(c) By adiabatic evaporation and cooling: By flashing a feed solution adiabatically to a lower temperature and inducing/causing crystallization by simultaneous cooling and evaporation of the solvent. 

(d) By adding a new substance that reduces the solubility of the original solute, i.e. by salting. 

When the solubility of the solute increases with an increase in temperature a saturated solution becomes supersaturated, i.e. supersaturation is generated by cooling and temperature reduction. This is the case with many inorganic salts and organic substances (e.g. potassium nitrate, potassium chloride, oxalic acid, etc.). 

Miers Supersaturation Theory

According to Miers Supersaturation theory, there is a definite relationship between the concentration and temperature at which crystals will spontaneously form in a pure solution. This relationship is represented by the super solubility curve which is approximately parallel to the solubility curve. Both curves are shown pic the. The curve AB is the solubility curve and the curve PQ is the super solubility curve. The curve AB represents the maximum concentration of solutions that can be achieved by bringing solid solute into equilibrium with a liquid solvent. If a solution having the composition and temperature indicated by point C is cooled in the direction shown by the arrow it first crosses the solubility curve AB and we would expect here crystallization to start.

Actually, if we start with initially unseeded solutions, crystal formation will not begin until the solution is supercooled considerably past the curve AB. According to Miers's theory crystallization will start in the neighborhood of point D and the concentration of the solution then follows roughly along the curve DE. For an initially unseeded solution, the curve PQ represents the limit at which spontaneous nuclei formation begins and consequently, crystallization can start. According to Miers's theory under normal conditions, nuclei cannot form and Crystallization can't then occur in the area between the solubility curve and the super solubility curve i.e. at any position short of point D along line CD. 

>Gas Saturation and Types of Saturation in Gas

Take these Notes is, Orginal Sources: Unit Operations-II, KA Gavhane