FUNDAMENTAL STUDY ON PURIFICATION OF ORGANIC COMPOUNDS BY CRYSTALLIZATION WITH APPLYING ELECTRIC FIELD

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Zone melting is one of the most efficient methods to get ultrapure organic compounds. However, it has not yet been applied to separation and purification processes in industrial scale, because the rate of separation is slow. In this work, the effect of an electric field on normal freezing was investigated experimentally, in order to examine whether an electric field can improve the separation efficiency or not. In this work, purification of 3,5-xylenol+naphthalene system was attempted by normal freezing. This is a simple eutectic-forming mixture. The effect of an electric field on the separation efficiency was experimentally examined by applying an electric field in parallel with the solid-liquid interface during solidification at 6,000 V/cm or 10,000 V/cm. The initial impurity concentration was 5wt%. After the solidification, The concentration of the impurity for the solid sample was analyzed by a gas chromatography. Furthermore, the solidification front and the flow pattern in the liquid phase were also observed by using a microscope with a video camera. When 3,5-xylenol was the main component, the concentration of the impurity (naphthalene) was decreased by an electric field. This may due to the convection in the liquid phase induced by the electric field, which was clearly observed through the microscope. On the other hand, when naphthalene was the main component, the concentration of the impurity (3,5-xylenol) was increased by an electric field. It was increased with increasing the strength of electric field. In this case, a lot of small crystals were observed through the microscope when the electric field was applied, so that 3,5-xylenol (impurity) may be adsorbed much more on grain boundaries.