Diffusion Driven Desalination for Simultaneous Fresh Water Production and Desulfurization

It is well recognized that oceans cover about 70% of the earth’ssurface and account for 97% of its water while only 3% of allwater on earth is fresh. Most fresh water is locked away in theform of ice caps and glaciers located in the Polar Regions. As aresult only 1% of all the water on earth is accessible for humanconsumption. Due to steadily growing demand, there is a need fornew fresh water resources. The obvious choice for this prospec-tive new water resource is the 97% of available water in theoceans, which is saline and requires desalination for use in bio-logical, agricultural, and industrial applications. Desalination in-volves any process in which dissolved minerals are removed fromsaline or brackish water. Technologies developed for desalinationapplications include thermal distillation, reverse osmosis RO ,electrodialysis ED , and vacuum freezing. Distillation and ROare the most commercially prominent. Thermal distillation tech-nologies include multiple effect distillation MED and multistageﬂashing MSF . Although MED, MSF, and RO technologies cur-rently provide the most cost effective means for desalination, theirdrawback is that they are very energy intensive, and whether ornot they remain cost effective strongly depends on energy pricesand energy supply.According to the International Desalination Association work-ing paper 1 , there were more than 17,348 industrial scale desali-nation plants installed or contracted in 2004. The total productionfrom these plants is on the order of 8.3 910 imperial gallons perday. RO provides 47.2% of the total capacity. The leading thermalprocess, MSF, provides 36.5% of the total capacity. The remainingare divided between other thermal processes such as, ED, vaporcompression VC , multiple effect evaporation MEE , and othernewer concepts. The main sources of feed water for desalinationare seawater at 58%, brackish ground water at 23%, and riverwater at 7%.In the last two decades, a new desalination technology hasdrawn considerable interest and is referred to as the humidiﬁca-tion dehumidiﬁcation HDH process. It operates on the principleof mass diffusion and utilizes dry air to evaporate saline water,thus humidifying the air. The water vapor in the humidiﬁed dry airis then condensed, producing fresh water as a result of dehumidi-ﬁcation. The greatest advantage of this technology is that it oper-ates under low pressure and low temperature and, as such, has thepotential to be both energy and capital efﬁcient. Bourouni et al. 2 , Al-Hallaj et al. 3 , and Assouad et al. 4 , respectively, re-ported the operation of HDH units in Tunisia, Jordan, and Egypt.Another type of desalination technology that makes use of waterevaporating into an air stream is the carrier-gas process CGPreported by Larson et al. 5 . A comprehensive review of solardriven HDH processes was reported by Al-Hallaj and Selman 6 .They concluded that solar driven HDH processes are not eco-nomically competitive with conventional MED, MSF, and ROtechnologies.Klausner et al. 7 described a type of HDH process, diffusiondriven desalination DDD , which is best suited to be driven bylow-grade waste heat. The DDD process has been extensivelystudied in an operating conﬁguration, where heated water andambient air are input to the diffusion tower 8 and direct contactcondensation 9 is used to produce distilled water. The DDDprocess is in some ways similar to the HDH and CGP processes inthat the mass diffusion of water molecules into low humidity air isthe driving mechanism to evaporate saline water. Because the de-salination is accomplished at relatively low temperatures, inex-