Crystallisation Rate of Pentaerythritol

Abstract

The growth rate of Pentaerythritol crystals in aqueous solution has been measured over the temperatures range 30 to 75°C by following the decrease in solution concentration of stirred crystal suspensions by refractometry. Measurements have also been made of the crystal mass increase in a fluidised bed for comparison at low relative crystal/solution velocities. Two methods of crystal size analysis have been used: the Coulter Counter method and the image shear microscope method. The Coulter Counter theory has been corrected to allow for crystal shape and size, and the seed size distributions determined allowed for in the computation of crystal growth rates. Commercial material containing two major impurities Di-P.E. and a Formal, exhibited an unusual transient overlap effect in the equilibrium solubility determination. The solubility approached from dissolution first obtained a peak enhanced concentration before attaining equilibrium. The equilibrium relationship for purified material was found to be represented by: Log10 x = 4.980 – 1242/T where x = % mass fraction and T is in o K. Growth rates (g), which ranged from about 10-8 cm/min to 10-3 cm/min were correlated with supersaturation (s) by the equation g = KLsb. It was found that b varied with the amount of impurity and temperature but had an average value of ca. 2. Values of kL increased with temperature and the activation energy for commercial material was found to be about 30 kcal/gmol. It was concluded that surface integration was the rate controlling process. Heterogeneous particles were found to enhance growth rates but the absence of particles >0.45 micron resulted in brittle crystals. Although it was found that purification from Formal also enhanced the growth rate, the rate was very sensitive to traces ( <0.1ppm level) of an unidentified third impurity which could not be easily removed.