FREEZING OF A SALT SOLUTION IN A PACKED-BED

There are many natural processes and technological applications that i nvolve the solidification of a binary solution saturating a porous mat rix. Some of them are. natural freezing and artificial freezing (for c onstruction purposes) of soil, oil exploration in cold regions, and pr ocessing and preservation of food. This paper presents the results of a fundamental study of freezing of a binary salt solution saturating a packed bed An aqueous sodium chloride solution (of noneutectic compos ition) constituted the binary solution and spherical glass bends const ituted the packed bed. The freezing was initiated at one of the vertic al walls of a rectangular cavity.;v. The temperature distributions in the solid mush, and liquid regions were recorded using thermocouples. The concentration of salt was determined using a sample withdrawal tec hnique in conjunction with a refractometer and a calibration chart. Th ere was buoyancy-driven convective flow generated and sustained by the thermal and solutal gradients. The effect of this flow on the freezin g process was significant. The morphology of the freezing fronts, the temperature and salt concentration profiles, and the rate of freezing were all influenced by the flow. Even in experiments with an initial s uperheat of 10 degrees C, it was found that the effect of flow was con siderable. For even though the fluid flows through the interstitial sp aces in the porous matrix, the permeability was large for balls of 0.5 -in. diameter. With a superheat of 20 degrees C, the convection was vi gorous and the rate of freezing was retarded considerably. The salt re jected during freezing was redistributed by the flow. At later times, a stable solute-rich region formed at the bottom of test cell where th e concentration decreased with height. The amount of salt rejected was directly influenced by the rate of freezing, which in turn was contro lled by the superheat and the permeability of packed bed.