Direct contact ice crystallization process: Modeling and simulation

The modeling and simulation of the direct contact ice crystallization process have been carried out in order to evaluate the particle size distribution (PSD) of the frazil ice crystals produced using an insoluble evaporating refrigerant in direct contact with water or sea water. This has required the solution of the material, energy and population balances, as well as a special consideration of the refrigerant droplet-crystal interface. For the growth and nucleation functions that are part of the population balance, updated models have been developed and used.; Previous works about continuous and batch crystallizers as well as computer simulations of ice formation in natural water bodies have been taken into account. Rapid growth of the nuclei and absence of very small particles are reported in the experimental works, while high concentrations of small particles are found in the theoretical works.; It is well known that frazil ice crystals develop in turbulent supercooled water. These flat disc shaped crystals are common in rivers, large lakes and in the ocean. The production of frazil ice in crystallizers to obtain either fresh water or “fluid ice” using a refrigerant in direct contact with the saline water is very similar to the generation of these ice particles in the ocean. However, in the crystallizer the ice crystals are thicker and the population of crystals per unit volume can be much larger. Another important difference is that in a crystallizer the region where the refrigerant liquid droplets are evaporating is a region where the water is exposed to a colder temperature than the rest of the bulk solution. For analysis, it is convenient to split the volume of the crystallizer into two sectors, a small one where the supercooling is high, this is called the “Cold Spot Region”, and a larger one (bulk solution) with smaller or very small supercooling. The Cold Spot Region (CSR) is a new concept, which holds that as nuclei are entrained at the refrigerant droplet-brine interface there is rapid ice crystal growth. The contact time between the nuclei and the liquid refrigerant in this CSR is obviously affected by fluid turbulence.; According to experimental results, the secondary nucleation process develops only in the CSR. The nuclei generated in the bulk solution cannot survive because of the very small supercooling. In the CSR nuclei and crystals grow in a process driven by large supercooling, while in the bulk solution only crystals grow under small supercooling. The frazil crystals produced in a crystallizer are thicker than the frazil particles generated in the ocean; this is because of the more frequent collisions of the crystals in a crystallizer, and this higher frequency is in turn a consequence of the higher concentration of particles. Considering this, the growth process for the frazil ice particle has been modeled by considering that the radial and thickness growth are interrelated.; Using the Cold Spot Region concept in the crystallizer analysis, it is possible to explain experimental results and to obtain particle size distributions in good agreement with experimental data. The new concept is also useful in the simulation of the frazil ice formation in natural water bodies.