Abstract
In freezing water which contains solutes, the solutes are removed from the frozen phase and concentrated in the unfrozen liquid phase, and strong agitation of the freezing interface is very effective. We used a supersonic radiation method instead of propeller agitation.
The experiments of freeze concentration with/without supersonic radiation were carried out under a constant freezing rate (40 mm/h), using two concentrations (0.5, 5.0 kg/m3) of three aqueous solutions (sodium chloride, L-phenyl alanine and saccharose). Under this freezing rate, freezing without supersonic radiation could not concentrate solutes, but freezing with supersonic radiation could greatly do, and decreased the average distribution factor under 0.4. The distribution factors at a late stage of freezing are much smaller than those at an early stage, because the turbulence of solution by supersonic cavitation and the agitating intensity per unit unfrozen volume increases. Therefore the solutes are not easily caught on the freezing interface as the freezing interface approaches the supersonic radiation horn. The low mass concentration solution is concentrated more than high concentration solution. For a constant mass concentration solution, the solutes of large molecular weight are more easily separated and concentrated than those of small molecular weight.
