Reverse osmosis desalination process: water permeability constant assessment

There are some major desalination processes available those are reverse osmosis, electro dialysis, freezing and distillation. The range of applicability of one process over the other is determined primarily by the salinity and composition of the feed water. As previous said, desalination processes can be based on thermal or membrane separation methods and the thermal processes separate to two categories, which are:
* Evaporation followed condensation of the formed water vapor
* Freezing followed by melting of the formed water ice crystals.

In addition, if the evaporation process takes place over a heat transfer area it is called as boiling, or within the liquid bulk, it is called as fashing. The evaporation processes include single effect vapor compression (SEE), multiple effect evaporation (MEE), multistage fash desalination (MBF), humidification-dehumidification (HDH) and solar stills. The differences between humidification, dehumidification and solar from other evaporation processes are the following:
* The main driving force for evaporation is the concentration difference of water vapor in the air stream.
* The water evaporate temperature is lower than the boiling temperature.

Furthermore, the single effect vapor compression includes thermal vapor compression (TVC), mechanical vapor compression (MVC), adsorption vapor compression (ADVC), absorption vapor compression (ABVC) and chemical vapor compression (CVC). Solar energy can also be used to desalt water directly in solar stills or used as an energy source for other thermal processes for example hybrid desalination system. Vapor compression is combined with the single or multiple effect desalination units to improve the process thermal efficiency.

This technique is meant that the low temperature vapor formed in the same effect or the last evaporation effect is compressed to a higher temperature and is then used to derive or initiate the evaporation process in the first or the same evaporation effect. The vapor compression devices include steam jet ejector if called as thermal vapor compression, mechanical compressors, adsorption, desorption beds and absorption, or desorption columns. On the other hand, the main membrane desalination process is reverse osmosis (RO) which will discuss deeply in later chapters. In a brief description, RO process is meant that fresh water permeates under high pressure through semipermeable membranes leaving behind highly concentrated brine solution. There is another membrane process called electro dialysis (ED) which is very limited used in industrial. This process separates the electrically charged salt ions through specific ion exchange membranes leaving behind low salinity permeate water. Consequently, a highly concentrated brine stream is formed on another side of the membrane. (Porteous, 1975, El-Dessouky and Ettouney, 2002)