Desalination Breakthrough: Saving the Sea from Salt – Scientific American
The secret is a variant of the Solvay process, a 150-year-old, seven-step chemical conversion method that is widely used to produce sodium carbonate for industrial applications, and that many chemists are working to refine. Benyahia has simplified the process in part by aiming for sodium bicarbonate (baking soda) rather than sodium carbonate, thus reducing the needed chemical conversion steps to just two. In the presence of ammonia he reacts pure carbon dioxide with the waste brine from desalination, creating solid baking soda and ammonium chloride solution. In a second step he reacts the ammonium chloride solution with calcium oxide to produce calcium chloride solution and ammonia gas. Recovering the ammonia allows him to reuse it in the first step, reducing the cost of the process.
Benyahia’s process is unusual in that it reduces the need for brine disposal by nearly 100 percent, ending up with sodium bicarbonate, calcium chloride and ammonia for reuse in the first step. It also uses pure CO2, whereas other similar processes use flue gas from power plants—which is about 10 percent CO2 and contains other gases. Using flue gas adds a step of separating out the pure CO2, making the process more expensive. Qatar already has natural gas processing plants venting pure CO2 close to brine disposal stations, making Benyahia’s solution potentially cost-effective, at least in places with similar infrastructure.
Brine disposal is a big problem in much of the Middle East. The gulf, along with the Red and Mediterranean seas, are turning saltier because of desalination by-products—and the region is the epicenter of desalination worldwide, with the United Arab Emirates, Saudi Arabia, Kuwait, Qatar, Bahrain and Oman making up 45 percent of global desalination capacity. This brine is typically twice as salty as seawater, and advanced desalination plants still produce approximately two cubic meters of waste brine for every one cubic meter of clean water.