Progressive Freeze-Concentration of Aqueous Salt of Fatty Acid in Annular Cylindrical Cooling Apparatus

Using an annular cylindrical cooling apparatus (ACCA) composed of a rotating inner cylinder and a stationary outer cylinder, we applied progressive freeze-concentration (PFC) of an aqueous solution of sodium oleate (NaOl). We focused on the growth of ice on the inner cylinder and the distribution of aqueous solute at the growth surface. Initial ultrasonic irradiation of the solution minimized supercooling until the growth of the ice layer began. Changing the rate of rotation revealed that distribution of the aqueous solute into the ice layer could be suppressed by Couette–Taylor (CT) flow, i.e. toroidal vortices periodically arranged and coaxially stacked in the annular space. By measuring the thickness of the ice layer and the concentration of the solute in the condensate in the presence of CT flow, we revealed the relation between the growth rate of the ice layer and the effective distribution coefficient of the solute: slow cooling reduced the distribution of the solute into the ice. Comparison with results using sodium octanoate (NaOc) and sodium propionate (NaPr) as model solutes showed that the effective distribution coefficient increased in the order of NaPr < NaOc < NaOl, suggesting that a solute with a greater formula weight was more easily trapped into the ice layer.