An Investigation Into the Use of Fluorinated Hydrating Agents In the Desalination of Industrial Wastewater

 

Salts are removed from industrial wastewater due to their fouling and equipment corrosion effects.  Current desalination technologies include reverse osmosis, multi stage flash distillation, vapour compression, electrodialysis and thermal distillation.  The current desalination technologies are sufficient in retrieving pure water from a low solute concentrated feed, however such processes are energy intensive, with scaling of saturated sulphates and membrane damage due to the presence of chloride (Khawaji et al., 2008).  There is therefore a need for recovering water from the concentrated brine solution at ambient temperatures and pressures.  Membrane distillation and gas hydrate technology are currently being investigated (Osman et al., 2010) as suitable feasible alternative technologies.  During the process of membrane distillation, crystal disengagement and a rapid decline in the distillate flux may occur, resulting in the flux tending to zero (Mariah et al., 2006). As a result, desalination using gas hydrate technology, in particular using hydrofluorocarbons as formers is currently being investigated. Hydrates are composed of guest molecules, hydrofluororcarbons, and host molecules, water.   When hydrates form, normally under high pressures and low temperatures, the host molecules form a cage, enclosing the guest molecule. This guest molecule may be in the form of a gas or a liquid. Desalination using gas hydrate technology includes the formation of hydrates by mixing industrial wastewater and the former at the hydrate forming temperature and pressure. Hydrate crystals will form leaving a concentrated salt solution. The solution will be filtered or decanted and the hydrate crystals will be washed to remove excess salts attached to the crystals. The former will then be separated from the fresh water and re-injected into the feed, while the fresh water can be reused and the salts recovered (McCormack and Anderson,1995).  The use of hydrofluorocarbons as hydrate formers reduces the hydrate dissociation pressure while increasing temperature.  Criteria for choosing the best fluorine formers require the former to be environmentally acceptable; non-toxic; non-flammable; chemically stable; a class 2 hydrate; available in commercial quantities; low cost; compatible with standard materials; as well as contain a high critical point (McCormack and Anderson, 1995).  Thus R22, and R134a were chosen as possible formers and will each be investigated in the presence of various concentrations of NaCl and CaCl₂.  According to Javanmardi and Moshfeghian (2003), to enable gas hydrate technology to be used as a competing desalination technology, energy costs must be reduced.  A gas hydrate promoter, cyclopentane, which will allow hydrates to form at ambient temperatures and moderate pressures will be investigated in the presence of the fluorinated refrigerants and various salts.  These systems will be measured using a static high pressure equilibrium cell (Tshibangu, 2010) and the results will be modeled.  The hydrate phase will be modeled using the method developed by Eslamimanesh et al. (2010), while the liquid phase will be modeled using the Aasberg Peterson method (Aasberg Peterson et al., 1991).

References

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