High-alloy stainless steel in seawater desalination processes
Seawater desalination is an industrial process used to obtain water for drinking , irrigation or industrial use . Desalination can be achieved using various methods : the most common exploit well-known natural phenomena such as evaporation or osmotic pressure of a saline solution .
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Evaporative desalination is carried out thermally by evaporating seawater . The resulting condensation of the steam produced is practically free of salts . Two different industrial processes are used for this method : Multi-Stage Flash ( MSF ) or Multi-Effect Distillation ( MED ).
MED and MSF plants Multi-Effect Distillation plants exploits the phenomenon of the osmotic pressure of two solutions with different saline concentrations on either side of a membrane which is permeable to water molecules and impermeable to dissolved salts . Seawater is pumped against these membranes ; water passes through and is collected and the salts which cannot pass through become concentrated and removed . This process has increased in popularity since the 1980s . It is still the subject of studies to improve the quality of the membranes by optimising yield . Multi-Stage Flash desalination plants ( Figure 1 ) consist of several evaporation stages which generally vary , depending on the desired performance , from a minimum of 15 to a maximum of 20-22 . In these desalinators , preheated and deaerated seawater passes through the tube bundles installed in the evaporation chambers . Flowing inside the tube bundle , on the outside of which the released steam condenses , the water recovers energy by heating itself . Once it has come out of the tube bundle that works at the highest temperature ( 1st stage ), the water is brought to the required temperature with the brine heater and then fed into the first evaporation chamber . Each evaporation stage is connected to a vacuum system capable of maintaining pressure inside
the stage which is slightly lower than the equilibrium pressure at the operating temperature . The water evaporates , and the vapour is condensed outside the tube bundle and then recovered as a distillate . The process is repeated in all subsequent stages , which work at gradually decreasing temperatures and pressures . Part of the brine is extracted and discharged from the last evaporation chamber , while a portion
Figure 1 . Schematic of a Multi-Stage Flash desalinator .
Figure 2 . Reverse osmosis desalination plant scheme .
is recovered , mixed with preheated seawater and put back into circulation . A Multi-Stage Flash desalinator requires about 6-9 m 3 of seawater for each cubic meter of distilled product . Multi-Effect Distillation plants contain a number of evaporation chambers ( called ‘ effects ’) which vary according to the desired performance . The evaporation chambers are connected to a vacuum system , which maintains a pressure slightly lower than that of equilibrium at the operating temperature inside each chamber . The seawater is superheated with respect to the operating temperature of the chamber into which it is injected . It releases steam which is condensed and collected in the form of distillate . A Multiple Effect Desalinator must be fed with approximately 5-6m 3 of sea water for each cubic meter of distilled product .
28 Stainless Steel World November 2022 www . stainless-steel-world . net