TECHNICAL AND ECONOMIC EVALUATION OF DESALINATION METHODS FROM SEAWATER

One of the biggest problems in the world is the lack of suitable water for drinking, which has several reasons for this group of problems. While ۷۲% of the planet is covered by water, the presence of salt, salts and minerals in ۹۲% of them has caused a shortage of fresh water in most parts of the world. In addition to water treatment and reuse, desalination or desalination of water is one of the solutions that is provided for this purpose and it tries to use various techniques of salt, salts and minerals in brackish sea water. Removed and finally used as fresh water. The output of these processes is fresh water and salt separately. Numerous methods have been adopted to solve the problem of water shortage, including dam construction, cloud fertility, and seawater desalination. But since the only endless source of water is the oceans and seas, the best way to develop and advance water supply is to desalinate seawater [۱]. Due to the great importance of water desalination processes, several methods are used for thispurpose, each of which has its own advantages and disadvantages. The most common methods of water desalination can be mentioned as follows: a) Thermal desalination methods; As the name implies, these processes separate salts and salts from water by heating them. These methods are often used for desalination of seawater and will not be economically viable for desalination of waters with less than ۳%. Thermal methods of desalination of water include the following: b) Multistage Flash Distillation (MSF); One of the most common and oldest methods for desalination of seawater is the use of multi-stage flash distillation, which is based on rapid distillation or flash to reduce water pressure. With the help of MSF, water desalination tanks are able to desalinate ۵۰,۰۰۰ to ۶۰,۰۰۰ cubic meters of seawater daily by heating ۹۰ to ۱۲۰ ° C. The basis of this method is that first the water enters the heating tanks and its temperature reaches ۹۰ to ۱۲۰ degrees Celsius and then it enters the next stage and its pressure is drastically reduced. Reducing the water pressure eventually leads to rapid evaporation in the form of flushing. The evaporated water will condense on the pipes of the heat exchanger, and in the next step, the pressure drop will increase and evaporation will take place again. The number of steps of this method can be implemented from ۴ to ۴۰. The amount of water remaining in the tank can be used for use in heat exchangers, in which case it is called recycled multi-stage distillation. In addition, in some cases, water is used only once and after desalination, the remaining water is drained from the tank. Despite the high efficiency in desalination of water with this method, there are some disadvantages in it, which are: a) Costly design and setup. b) Excessive deposition on stainless steel pipes. Other methods of water desalination can be referred to multiple effects or Multiple-effect distillation, which by heating salt water and reducing its pressure in different stages, eventually a large volume of salt and other salts are removed. Desalination of water by distillation of several effects is such that first steam with high temperature enters the heat pipes and feed water in the initial stage is in contact with these pipes and evaporates. The feed water enters the second stage after evaporation and the saline water exchanges temperature with the existing water vapor as soon as it enters this stage and turns into steam. By evaporating salt water, a large amount of salt and its salts are removed and enter the next stage. The steam pipes considered in this method are designed intentionally or horizontally and the steam in them returns to the boiler after cooling due to the temperature exchange with the feed water. The desalination method with the help of multi-effect distillation does not require pretreatment steps and has low set-up and maintenance costs, but there is sedimentation withincreasing temperature, which is considered as a limitation.