Salts in solution should be removed by desalination techniques to prevent equipment fouling and
corrosion. Common desalination technologies are energy intensive such as Multi Stage Flash
(MSF) distillation which requires 14.5 J/m3 (Ribeiro. J, 1996) of energy. Desalination technologies
produce purified water and a concentrated salt solution, where the salt concentration is dependent
on the desalination technology used. This work investigates gas hydrate technology as a possible
desalination technology.
Hydrates are composed of guest molecules and host molecules. Guest molecules may be in the
form of a liquid or gas. During hydrate formation, host molecules, water, form a cage enclosing the
guest molecule. Common hydrate formers or guest molecules such as; methane, ethane, propane
and carbon dioxide are currently being investigated in literature, for use in gas hydrate desalination
technology. Common hydrate formers form hydrates at low temperatures; below 288 K and high
pressures; above 2 MPa. To increase the temperature and reduce the pressure at which gas hydrates
form, commercially available hydrofluorocarbon hydrate formers such as R14, R32, R116, R134a,
R152a, R218, R404a, R407c, R410a and R507 are preliminarily investigated in this work.
The criteria for choosing the most suitable fluorine-based formers require the former to be:
environmentally acceptable where it is approved by the Montreal Protocol; non-toxic where it has a
low acute toxicity; non-flammable; chemically stable; a structure II hydrate to simplify the washing
process; available in commercial quantities; low cost in comparison to other hydrate formers;
compatible with standard materials and contain a high critical point for a large heat of vaporisation
(McCormack and Andersen, 1995). Taking all these criteria into account, R134a was chosen for
further investigation as a possible hydrate former.
In this work, hydrate-liquid-vapour phase equilibrium measurements are conducted using the
isochoric method with a static high pressure stainless steel equilibrium cell. The Combined
Standard Uncertainty for the 0-1 MPa pressure transducer, 0-10 MPa pressure transducer and the
Pt100 temperature probes are ±0.64 MPa, ±5.00 MPa and ±0.09 K respectively. Vapour pressure
measurements for Hydrofluoropropyleneoxide, CO2, R22 and R134a were measured to verify the
pressure and temperature calibrations. Hydrate test systems for R22 (1) + water (2) and R134a (1)
+ water (2) were measured to verify calibrations, equipment and procedures. New systems
measured included R134a (1) + water (2) + {5wt%, 10wt% or 15wt%} NaCl (3).
For the system R134 (1) + water (2) at 281 K the dissociation pressure is 0.269 MPa. However,
addition of NaCl to the system resulted in a shift of the HVL equilibrium phase boundary to lower
temperatures or higher pressures. The average shift in temperature between the system R134a (1) +
water (2) containing no salt and the systems containing {5, 10 and 15} wt% NaCl are -1.9K, -4.8K
and -8.1K respectively.
In this work, the measured systems were modelled using two methods of approach. The first
method is where hydrofluorocarbon hydrate former solubility is included, (Parrish et al., 1972) and
the second is where hydrofluorocarbon hydrate former solubility is ignored, (Eslamimanesh et al.,
2011). From these models, it is found that hydrofluorocarbon solubility could not be neglected.
In this work, the hydrate phase was modelled using modifications of the van der Waals and
Platteeuw model, (Parrish et al., 1972). The liquid and vapour phases are modelled using the Peng-
Robinson equation of state with classical mixing rules (Peng, 1976). The electrolyte component is
modelled using the Aasberg-Peterson model (Aasberg-Petersen et al., 1991) modified by Tohidi
(Tohidi et al., 1995). The percent absolute average deviation (%AAD) for the systems, which
includes solubility, is 0.41 for R22 (1) + water (2) and 0.33 for R134a (1) + water (2). For the
system R134a (1) + water (2) + {5 wt%, 10 wt% or 15 wt%} NaCl (3) the % AAD is 5.14.
Using the hydrate former, R134a, is insufficient to ensure gas hydrate technology is competitive
with other desalination technologies. Hydrate dissociation temperature should be increased and
pressure decreased further to ambient conditions. As evident in literature, promoters, such as
cyclopentane, are recommended to be added to the system to shift the HLV equilibrium phase
boundary as close to ambient conditions as possible. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2011.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/8986 |
| Date | January 2011 |
| Creators | Petticrew, Cassandra. |
| Contributors | Ramjugernath, Deresh D., Naidoo, Paramespri., Mohammadi, Amir H., Buckley, Christopher A. |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
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