Measurements of the solubility and metastable zone for the oxalic acid-water system were obtained. The solubility was measured within the temperature range from 284.19 to 352.02 K. The mole fraction solubility was correlated satisfactorily with the temperature by the equation: x eq ) 4 × 10 -7 e 0.0368T . The values of enthalpy of dissolution, enthalpy of fusion, and enthalpy of mixing were determined to be 30.80, 58.158, and -27.358 kJ mol -1 , respectively. The high value of enthalpy of mixing indicates a strong interaction between the solute and the solvent molecules. The width of the metastable zone was measured by an ultrasonic method and was correlated with the cooling rate and the equilibrium temperature by the equation: ∆T max ) 7.2206 × 10 15 b 0.266 T eq -6.17 . The nucleation parameters of oxalic acid in water were determined from the metastable zone data. Over the equilibrium temperature range from 294.75 to 320.75 K, the nucleation rate constant was varied from 0.0084 to 0.0628 #/m 2 ‚min, whereas the nucleation order was varied from 3.2419 to 2.8292. The obtained higher values of nucleation rate constant indicate a higher rate of nucleation. The role of the presence of small amounts of sulfuric acid on the metastable zone width was also investigated. The widest metastable zone was measured due to the presence of sulfuric acid at a concentration of 2.3 × 10 -9 g/g of solution.

The effect of the type of solvent on the solid liquid interfacial energy was determined by performing induction time measurements of paracetamol in methanol, 1-propanol, acetone and water at a constant supersaturation temperature of 30°C and different levels of supersaturation (a/a*) ranging from 1.03 to 1.24. At equal supersaturation level and temperature the induction time increases with decreasing solubility whereas the solid liquid interfacial energy decreases with increasing solubility. The interfacial energy has a minimum value of 1.45 mJ/m 2 in the solvent where paracetamol has a maximum solubility (methanol) whereas it has its maximum value of 2.91 mJ/m 2 in the solvent with minimum solubility. The interfacial energy is a function of the solubility has been established. The critical radius for homogeneous nucleation was found to be minimum in the solvent of highest solubility.

Growth rate of potassium sulfate crystals was studied in a fluidized bed crystallizer. Higher growth rates were observed at larger bed heights. Larger crystals grow faster than smaller size crystals due to the changed fluid dynamics. The growth rate was found to be very sensitive to the pH value of the solution. The growth rate increases with increasing the pH value and the increase of growth in the acidic medium is faster than in the basic medium. The presence of small amounts of Cr 3+ ions reduces the growth rate of potassium sulfate dramatically. The presence of Cr 3+ ions lowers the saturation temperature and increases the width of the metastable zone, i.e. shifts the metastability of the solution to be at lower level of supersaturation.

An ultrasonic technique is used to develop a method based on the measurement of both ultrasonic velocity and temperature of the supersaturated media for the on-line determination of the supersaturation. It is demonstrated by the example of aqueous K 2 SO 4 solution in a seeded batch crystallizer. The feasibility of this technique was investigated using a commercial available sensor. The experimental measurements for the desupersaturation versus time trajectory proved that this technique can be successfully employed to measure supersaturation with sufficient accuracy and precision. Such sensor is preferred in the on-line application due to its fast response and the availability to be applied directly in the slurries without any need to the solid-liquid separation.