Freezing Methods



Much like the solar radiation method,the conversion of salt water to fresh water has always existed in nature. It was once used by inhabitants of Central Asia and Western Siberia for generations to obtain water for their cattle. Short frost periods were used to free salt water from ditches where it was then allowed to melt in the sunlight. In open waters, some of the salts initially begin to freeze with the water. However, over time, mass transport takes place throughout the semisolid layer as solidification continues to take place. As this occurs the salinity throughout the layer decreases with time. As a result, glacial and sea ice are relatively salt-free. Some have even gone as far as to propose that icebergs be towed from the Antartic to be remelted into fresh water.

As one may have noticed thus far, desalination processes fall into two categories: those that remove the fresh water, leaving concentrated salt water behind and those that remove the salts and leave behind the fresh water. The freezing method falls within the first category. Salt waters have a certain critical temperature which is a function of its salinity. When the salt water is reduced to this temperature, ice crystals composed of fresh water are formed. It is then possible to mechanically separate the ice crystals from the solution and remelte them to get fresh water. This is the basic principle on which freezing desalination methods are based on.

It is obvious that this method is not the most practical to use. It can be easily demonstrated on a small-scale basis but when applied to a large scale process, problems arise. The main problem lies in the economical aspects of the initial capital costs and the maintenance costs. The second problem is the thermodynamic effieciency relative to upscaling the process.

Believe it or not, there are some advantages to this process. The operating temperature of this type of process is obviously at or below the freezing temperature of water. At these temperatures, scaling and corrosion are greatly reduced. This is one of the drawbacks of other conventional methods. Scaling is due to the buildup of precipitates due to calcium, magnesium, bicarbonate, sulfate, sodium, and chlorine ions in the water. The hard scale formed on the inside of the pipes requires costly maintenance practices to remove. Corrosion of steel pipes in contact with salt water is also increased with temperature. Lower temperatures permit plastics and protective coatings on the steel pipes to prevent corrosive attack. The thermodynamic efficiency can also be increased due to the lack of heat exchangers required in heat driven desalination processes.

There are several different methods for the separation of the ice crystals from the liquid. The chosen method is dependent on the characteristics of the ice crystals. Two items of concern are crystal size and specific gravity. Filtration seems like it would be an obvious approach but it is actually impractical. It requires a slow, complicated filtration system and has not yet been applied to any system. A wash-separation method is a more reasonable approach. It takes the specific gravity into account. The solution flows up a screened or perforated column and a floating column of ice crystals is formed. Since the salt gets trapped in this ice column, it must be counter-washed by process water.