Theodoros Varzakas
4.2 CRYSTALLIZATION EQUILIBRIUM
Once crystallization is concluded, equilibrium is set up between the crystals of pure solute and the residual mother liquor, the balance being determined by the solubil-ity (concentration) and the temperature. The driving force making the crystals grow is the concentration excess (supersaturation) of the solution above the equilibrium (saturation) level. The resistances to growth are the resistance to mass transfer within the solution and the energy needed at the crystal surface for incoming molecules to orient themselves to the crystal lattice (Earle and Earle, 2004).
Solubility is a function of temperature. For most food materials an increase in temperature increases the solubility of the solute as shown for sucrose in Figure 4.1.
Pressure has very little effect on solubility.
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Solubility is usually measured as how many grams of solvent can be dissolved in 100 g of solute.
A saturated solution is one which contains as much solute as the solvent can hold. A supersaturated solution contains more dissolved solute than a saturated solution, that is, more dissolved solute then can ordinarily be accommodated at that temperature.
Two forms of supersaturation exist. Metastable (just beyond saturation) and labile (very supersaturated). Crystallization is normally operated in the metastable region.
Crystallization is possible and spontaneous in the supersaturated or labile zone, how-ever crystallization is possible but not spontaneous in the metastable zone.
In general, crystallization is achieved by cooling a solution (if supersaturation is a function of temperature), removal of the solvent by evaporation (where supersatura-tion is independent of temperature, e.g., common salt) and addisupersatura-tion of another solvent to reduce solubility (when solubility is high and the above methods are not desirable, or in combination with the above methods, or the new solvent is called the antisolvent and is chosen such that the solubility is less in this new solution than it was before).
During crystallization, the crystals are grown from solutions with concentrations higher than the saturation level in the solubility curves. Above the supersaturation line, crystals form spontaneously and rapidly, without external initiating action. This is called spontaneous nucleation (Figure 4.2). In the area of concentrations between the saturation and the supersaturation curves, the metastable region, the rate of
+ + + + ++
FIGURE 4.1 Solubility and saturation curves for sucrose in water. (Ref: http:/www.nzifst.
org.nz/ unitoperations/contegseparation10.html.)
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initiation of crystallization is slow; aggregates of molecules form but then disperse again and they will not grow unless seed crystals are added. Seed crystals are small crystals, generally from the solute, which then grow, by deposition on them, further solute from the solution. This growth continues until the solution concentration falls to the saturation line. Below the saturation curve there is no crystal growth; instead, crystals dissolve.
The degree of supersaturation of a solution is usually defined as S = CB/CS*, where CB is the total solute concentration and CS* is the saturation concentration of the solute in the solvent. The degree of supersaturation that can be achieved before any crystals will form depends on storage temperature, the nature of the solute and solvent, the presence of any contaminating materials, and the application of external forces (Kashchiev and van Rosmalen 2003; Lindfors et al., 2008).
A key variable during batch crystallization processes is the solution supersatura-tion which significantly determines the development of nucleasupersatura-tion and growth phe-nomena (Srisa-nga et al., 2006) and, consequently, the final crystal yield and size. It is well established that the rate of cooling directly affects both nucleation and growth kinetics.
Kubota et al. (2001) showed that seeding plays a key role in crystallization to control crystal size distribution (CSD).
Supersaturation is of great importance in sugar crystallization. It has a profound effect on product quality and on the cost of production; parameters which determine the chances of survival of plants all over the world. It is also well known that seed-ing is a very critical step of crystallization and, therefore, this step should be carried out in a reproducible, reliable way, that is automatically based on the on-line moni-toring of supersaturation. The SeedMaster optional software developed by Rozsa (2006), which can be run directly in the PR-01-S-type process refractometer of the K-PATENTS OY Company (Finland), addressed these needs and has already proved its worth in mills in the United States, Peru, Colombia, and Iran. Owing to the hard-ware limitations, it was designed to display and transmit supersaturation data only (4–20 mA standard current output), and to implement fully automatic seeding of
Supersat, ∆C = C – C*
Nucleation
Crystal growth
Temperature C
C*
Concentration (kg solute/100 kg solvent)
FIGURE 4.2 Concentration versus temperature showing regions of supersaturation, nucle-ation, and crystal growth.
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crystallizers based on supersaturation or density. SeedMaster 2 is a further step in establishing a new class of instruments for crystallization control. It has a wide range of advanced features implemented in dedicated hardware and software, capable to serve two crystallizers simultaneously. It uses liquid concentration data provided by refractometers from the K-PATENTS family of products, and data from any one of the existing sensors measuring massecuite solids content, density, or power/cur-rent consumption of the stirrer motor. Based on on-line calculations SeedMaster 2 provides data on six massecuite parameters (per pan), including supersaturation and crystal content for transmission to external control equipment. It can be used to implement automatic seeding on its own as well.