CRYSTAL SIZE CONTROL OF SULFATHIAZOLE USING HIGH-PRESSURE CARBON-DIOXIDE

The effect of the pressurization method of carbon dioxide on the cryst allization behavior and crystal size of sulphathiazole (SUT) was inves tigated. In the ''stepwise pressurization'' method exceptionally large pillar-like crystals of 2-6 mm were obtained as mainly a scaling on t he wall of the crystallizer. In the ''rapid pressurization'' method, c rystals with a size one third to half of that obtained in the stepwise method precipitated, indicating the accelerated nucleation rate by th e rapid increase of the supersaturation degree with a vigorous bubblin g. With the new method of ''two-step pressurization'', in the first st ep the nucleation is accelerated with a much larger pressure instantly created, and in the second step the growth rate is retarded with the lower pressure. By this method much more fine crystals (from a few ten s to several hundred micrometers) were produced and the scaling was su ppressed. In this method a large supersaturation degree at an interfac e between the gas (bubble) and liquid phase under a vigorous bubbling may play an important role in accelerating the nucleation. The average size of the crystals tended to become smaller with increase of the fi rst pressure and the expansion ratio at a decompression point, and it tended to get larger with increase of the second pressure. These resul ts show that the GAS method is very useful for the control of crystal size over a wide range.