Читать книгу Introduction to Desalination - Louis Theodore - Страница 19

Crystallization processes were also employed over 2,000 years ago. Today, these processes are important mass transfer operations that are often employed in the preparation of a pure product. In the process, a crystal usually separates out as a substance of specific composition from a solution of varying composition. Any impurities in the liquid (often referred to as the mother liquor) are carried in the crystalline product only to the extent that they adhere to the surface or are occluded (retained) within the crystals that may have grown together during or after the crystallization operation. The separation of a solid from a solution onto a crystal occurs only if there is a state of imbalance involving a mass driving force; namely, a decrease in chemical potential (or concentration) between the bulk of the liquid solution and the crystal interface. This effectively means that the solution must be supersaturated. There are several different ways that crystallization can occur. The four most often encountered in practice are (Theodore 2014):

1 Cooling.

2 Evaporation.

3 Cooling and evaporation (also referred to as adiabatic evaporation).

4 A salting out process.

Process 1 is the most commonly employed, provided the solubility of the component being crystallized decreases with decreasing temperature.

World-wide development of desalination techniques in the last half century has been driven out of necessity due both to water scarcity and population growth. The private sector has primarily led the investment in research and development since water has begun to be seen not as a commodity, but as a product to be sold at a profit. This development by the private sector has led to a significant drop in the cost of water generated using these desalination techniques.