(606e) 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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2.     
Eslamimanesh.A, Mohammadi.A, Richon.D, (2010), Thermodynamic Model for
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Fontainebleau

3.     
Javanmardi J, Moshfeghian, (2003), Energy consumption and economic
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4.     
Khawaji.A,
Kutubkhanah.I, Wie.J, (2008), Advances in seawater desalination technologies, Desalination
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5.     
Mariah.L, Buckley.C, Naidoo.P, Brouckaert.C, Ramjugernath.D, Curcio.E,
Driolo.E and Jaganyi.D, (2006), Thermodynamic Modelling of Vapour Pressures
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6.     
McCormack.A, Anderson.R, (1995), Clathrate Desalination Plant
Preliminary Research Study, Water Treatment Technology Program Report No. 5, Thermal
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7.     
Osman.M, Schoeman.J and Baratta.L, (2010), Desalination / concentration
of reverse osmosis and electrodialysis brines with membrane distillation, Desalination
and water Treatment, (24),
293-301

8.      Tshibangu, M. M. (2010).
Measurements of HPVLE data for fluorinated systems, MSc Thesis, University of
Kwa-Zulu Natal