Reverse osmosis process
Reverse osmosis is one of the most widely used methods of separation, which has many applications. Sewage sweetening, the separation of organic and toxic substances from industrial wastewater are two of the most important uses of this technology. Reverse osmosis is one of the main methods of sweetening water along with methods such as distillation, electrodialysis, no ionization, and … used in laboratory, urban and industrial scale.
The history of the use of reverse osmosis techniques dates back to the 1950s, when experiments were carried out by Reed and Burton at the University of Florida, and the first reverse osmosis membrane was made of acetate cellulose, and in the late 1960s this phenomenon was commercially used .
Theoretical principles of reverse osmosis
Recognizing the phenomenon of reverse osmosis requires identification of osmotic property. In general, the transfer of mass from one environment to another is due to the difference in concentration or, in other words, due to the difference in chemical potential.
Consider, for example, a blue dish that is thrown into a piece of crystalline salt. In the part of the container containing salt crystals, high salt concentration and water concentration are negligible, and in the other part the concentration of salt is very low and the concentration of water is very high. Due to the difference in concentration, salt crystal molecules begin to penetrate into the other part, which is water, and on the other hand, water molecules also penetrate the salt section. This continues until the concentration of water and salt in both parts continues. Because after that, the driving force, which is the same difference in concentration, does not exist between the two environments, in other words, the state of equilibrium is established.
The osmotic property is referred to as passing a solvent from the dilute solution to the concentrated solution portion from within a semi-permeable membrane. To illustrate this feature, consider the following figure representing a container containing two parts and a membrane. Water or a dilute solution of salt water in one portion, a concentrated solution of brine in the other part and a membrane that allows only water molecules to pass through. The membrane prevents the passage of salt ions due to its large size.
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Initially, the fluid level is in two equal parts. Due to the tendency to equalize concentrations in two parts, water molecules pass through the dilute solution portion of the membrane and penetrate into the concentrated solution. The level of fluid in the thick part increases. The flow of water molecules from the membrane is carried out so that the difference in the liquid level (ie, the pressure difference) in two parts prevents the movement of water molecules. The pressure difference in the membrane-separated part, which prevents pure water molecules, is called osmotic pressure.
Osmotic pressure causes balance in two parts. However, if a pressure above osmotic pressure enters a thick solution, a photo of the osmotic phenomenon occurs, that is, water molecules flow from the thicker solution to the more dilute solution. This phenomenon is called reverse osmosis.
The reverse osmosis phenomenon is used for water purification and, more generally, for the condensation and separation process. Osmotic pressure is directly proportional to the concentration of solutes present in the solution. For example, in aqueous solution of salt, increasing the concentration to the amount of each mg / L causes a high osmotic pressure of 0.01 psi. It should be noted that due to the passage of water molecules from the membrane, the concentration of ions behind the membrane is gradually increased, resulting in increased osmotic pressure.