TOPIC 1: Gas Hydrates
54
©H
ERIOT
-W
ATT
U
NIVERSITY B41OA December 2018 v3
In phase equilibrium calculations for gas hydrate systems, the minimisation of
Gibbs energy is essential because this determines which hydrate structure is
formed (sI, sII or sH).It is obvious for a single component gas which structure
forms, but for a gas mixture it’s not so clear cut.
It is simplest to think of the hydrate formation process as being composed of
two steps:
1. The formation of the water framework.
2. The gas molecules enter the framework.
This process is clearly hypothetical because the empty water framework (
β
)
is not thermodynamically stable. The change in chemical potential between the
hydrate (H) and the separate species (
α
) is
(
) (
)
α
β
β
α
µ
µ
µ
µ
µ
µ
−
+
−
=
−
H
H
……………
..
……
(1.18)
The first term on the right describes the stabilisation of the water lattice due to
the introduction of the gas molecules into the cages;
numerous models exist
for the calculation of this term (discussed later).
The second term describes the phase change of water, from liquid water or ice
to the hydrate (sI or SII) and can be determined in the usual way
(
)
RT
P
T
RT
,
µ
µ
µ
α
β
Δ
=
−
(
)
dP
RT
v
dT
RT
H
RT
P
T
RT
P
P
T
T
∫
∫
Δ
+
Δ
−
Δ
=
−
∴
0
0
2
0
0
0
,
µ
µ
µ
α
β
……………………
...(1.19)
In which
R
is the universal gas constant,
T
is temperature (K),
P
is pressure,
H
is enthalpy and
v
is molar volume.
The subscript “0” refers to a reference state and the symbol
Δ
refers to the
change between a pure water phase (liquid water or ice) and the hydrate
phase (either sI or sII). Values for these quantities are available in the
literature (Pedersen et al., 1989).
In a typical hydrate problem the following phases could be in equilibrium:
•
Water rich phase (which could contain, electrolytes, chemical inhibitors,
etc.).
•
Liquid hydrocarbon phase.
•
Vapour phase.
•
Hydrate phases (structures-I, II, and H).
•
Ice.
•
Salt (salt deposition, in the case of salting-out).
TOPIC 1: Gas Hydrates
55
©H
ERIOT
-W
ATT
U
NIVERSITY B41OA December 2018 v3
Therefore, potentially 8 phases could coexist (excluding
other solid phases,
such as Wax and Asphaltene). The problem is further complicated considering
the complex behaviour of the fluid and solid phases.
In general, all fluid phases are modelled by an equation of state, hydrate
phases are modelled by the solid solution theory developed by van der Waals
and Platteeuw (1959) and the many advancements
that have been made to
this theory (see Sloan and Koh, 2008 for details).
Various other techniques
are used for modelling the water rich, salts and ice phases.
The original model of van der Waals and Platteeuw (vdW+P) for calculation of
the
β
µ
µ
−
H
term is based on statistics similar to the adsorption of gas on a
solid surface:
(
)
∑
=
−
=
−
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