Search

A new freezing process for the desalination of seawater is being developed which utilizes a unique way of upgrading heat energy where there is no gas phase involved. This method takes advantage of the abnormal melting point curve of water. Water melts at a lower temperature under a higher applied pressure (that is, (dp/dT) melting 0). Due to this differnce a substance which melts at a temperature lower than the freezing point of an aqueous solution may melt at a temperature higher than the melting point of water at a sufficiently high pressure. Thus, a suitably selected working medium can be used to form a cyclic auxiliary system which can be incorporated with the main system to: remove the heat of crystallization of water in the partial freezing of an aqueous solution by melting the working medium at a low pressure, and to supply the heat for melting the ice by solidifying the working medium at a sufficiently high pressure.

Normal freezing of several organic systems exhibiting simple eutectic behavior has been studied experimentally, and the occurrence of constitutional subcooling has been clearly established. This phenomenon, previously observed in systems of metallurgical interest, results in the instability of a planar solid-liquid interface and leads to solute trapping. This trapping mechanism quantitatively explains why eutectic forming systems exhibit solid phase concentration profiles identical to those expected of systems which form solid solutions. The onset of constitutional subcooling can be correlated and reliably predicted by theoretical equations. Although the process of normal freezing with no liquid phase agitation would appear to be well described by a diffusion model, free convection induced by concentration gradients renders a boundary-layer model more appropriate when the liquid density of the solvent exceeds that of the solute and freezing is in an upward direction.