Freezing of a Salt Solution in a Packed Bed

Abstract

There are many natural processes and technological applications that involve the solidification of a binary solution saturating a porous matrix. Some of them are: natural freezing and artificial freezing (for construction purposes) of soil, oil exploration in cold regions, and processing 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 composition) constituted the binary solution and spherical glass beads constituted the packed bed. The freezing was initiated at one of the vertical walls of a rectangular cavity. The temperature distributions in the solid, mush, and liquid regions were recorded using thermocouples. The concentration of salt was determined using a sample withdrawal technique in conjunction with a refractometer and a calibration chart. There was buoyancy-driven convective flow generated and sustained by the thermal and solutal gradients. The effect of this flow on the freezing 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 superheat of 10°C, it was found that the effect of flow was considerable. For even though the fluid flows through the interstitial spaces in the porous matrix, the permeability was large for balls of 0.5-in. diameter. With a superheat of 20°C, the convection was vigorous and the rate of freezing was retarded considerably. The salt rejected during freezing was redistributed by the flow. At later times, a stable solute-rich region formed at the bottom of test cell where the concentration decreased with height. The amount of salt rejected was directly influenced by the rate of freezing, which in turn was controlled by the superheat and the permeability of packed bed.