2013 年 46 巻 11 号 p. 785-789
The purity of crystal beds was investigated in the suspension melt crystallization of an organic acid. Methacrylic acid was used as the target material, and a certain amount of methanol was used as the solvent. Phenol was added as the representative of impurities. Under the various initial methanol ratios that mean mole fraction ratios of methanol to methacrylic acid, the effect of a small amount of the maleic acid byproduct formed during the methacrylic acid synthesis was focused on in batch crystallizations. In the presence of maleic acid, the impurity phenol concentrations of obtained crystal beds were higher than in the absence of maleic acid, and the difference became large with decrease of initial methanol ratio. Similarly, in the presence of maleic acid, the adherence ratios of mother liquor on the crystal beds were higher. However, although the adherence ratios descended with decrease of initial methanol ratio at first, since then they demonstrated an increasing trend. In the high range of initial methanol ratio, it was presumed that the reduction of the adherence ratios was attributed to decrease of specific surface area of crystal beds. Meanwhile, it was presumed that the degradation of adherence ratios in the low range of initial methanol ratio was derived from the influence of crystal bed height. Therefore, the relationship between impurity concentration and slurry density that was associated with crystal bed height was examined. As the result, although impurity concentrations were almost constant in the low slurry density range, they demonstrated increasing trends with ascent of the slurry density at high slurry density range. Moreover, in the presence of maleic acid, the impurity concentration demonstrated higher tendency when they were compared under the similar slurry density condition. These results indicates that the impurity phenol concentrations of obtained crystal beds immediately after the solid–liquid separation from slurry was affected by the crystal bed height especially in the higher slurry density range, further, the dependency of those was higher in the presence of maleic acid.