Page 234 - Handbook Of Multiphase Flow Assurance

Hydrate stability and crystal growth 233

Thermodynamics of hydrate formation
Hydrate formation without inhibitors
The phase equilibria of gas hydrates are of the most industrial and academic interest.
Thermodynamic conditions at which hydrates form from pure water are usually described by
a pressure-temperature diagram. An univariant curve describes the equilibrium for hydrate
formation from binary mixture water-hydrate former. The univariant curve is prescribed by
the Gibbs's phase rule:
+
2 + C = F P (10.1)
where C = number of components, F = number of degrees of freedom, P = number of phases.
The three phase line (V, Lw, H) for Structure I hydrate of methane will serve as an example:
+
+
22(methanewater ) =+ + + ) (10.2)
13(vaporhydrate liquid
This gives only one degree of freedom for hydrate formation equilibrium.
A large amount of hydrate equilibrium data for natural gases was compiled by Sloan
(1990). He analyzed the sI and sII hydrate equilibria for pure hydrate formers and their mul-
ticomponent guest mixtures.
Data for sH hydrate of methane and adamantane was reported by Lederhos et al. (1992).
Data for other hydrate structures have not been reported in the recent literature.
Usually, as temperature of the system goes down, a lower pressure is required to form gas
hydrate. This can be shown from the Clausius-Clapeyron equation.
d ln P) / d(1 /T) =−∆ d H ( .
(
/ ZR)
where Δ d H is the enthalpy of hydrate dissociation. Hydrate consumes heat on dissociation
and Δ d H is greater than zero. Thus the slope of ln P against 1/T is negative.
This also can be explained from an entropic viewpoint. At lower temperature water mole-
cules vibrate less and become more ordered. The entropy of the system decreases. According
to the equation:
−
=
+
G UPVT S, (10.3)
if the volume stays constant and temperature decreases, then the pressure must also decrease
in the closed system (U = const) at equilibrium (G = const).
Hydrate formation with inhibitors
There are four basic methods of prevention of hydrate formation. These include removal
of water from the system, raising the system temperature above equilibrium, decreasing the
system pressure below equilibrium, and introduction of an inhibitor.
Thermodynamic inhibitors such as alcohols or glycols are widely used in gas and oil industry
to prevent hydrate formation. When an inhibitor is added to the gas-water system, lower pressure
is required to form hydrate compared to a system without an inhibitor, at the fixed temperature.
Makogon (1981, p. 133) reported that, “With an increase in concentration of alcohols in
water, a breakdown is observed in the structural organization of water and in the clathrate-
forming aggregates. As a result, the probability of hydrate formation is reduced”. This obser-
vation suggests that the thermodynamic inhibitors change the structure of water away from
that favoring hydrate formation as a part of their effect. A result from a neutron diffraction
study (Soper and Finney, 1993) of a 1:9 M ratio methanol-water mixture concludes that the