Page 242 - Handbook Of Multiphase Flow Assurance

Hydrate stability and crystal growth 241

We used the Clausius-Clapeyron equation and the hydrate heat capacity by Handa (1986)
in order to show that a smooth equilibrium line is expected with the absence of a phase tran-
sition in the region. Justification for use of this equation for univariant systems over a narrow
temperature range comes from the fact that Δ d H and z do not change rapidly with tempera-
ture. Total change in Δ d H over the studied temperature interval was 22% of the initial value.
Eq. (10.4) can be used to determine the enthalpy of dissociation of the hydrate systems, as
validated by Handa (1986). This equation shows that the slope of the logarithm of the hydrate
equilibrium pressure versus inverse equilibrium temperature is proportional to Δ d H. When a
plot of Δ d H against T is monotonic and continuous, then a plot of ln[P] against 1/T will have
no slope discontinuity. It is also known that.
ΔH depends on T as follows:
T 0
∆ HT () = ∆ HT ( ) + ∫ ∆C P dT (10.5)
d
0
d
T
From this dependence it is seen that if a plot of ΔC p against T is smooth, then, so is Δ d H
against T in the absence of a phase transition. ΔC p was evaluated using the stoichiometric
formula:
P (
P (
∆C = C Hydrate) − 6 C Ice) − C ( Gas) (10.6)
P
P
assuming that the unit cell of sI hydrate crystal is formed by six ice molecules and one gas
molecule at the most probable 96% occupation of cavities by guest molecules. C p (Hyd) values
were taken from Handa (1986), and C p (Ice) and C p (Gas) were extrapolated from literature
values (Handbook of Chemistry and Physics, 1988; Friend et al., 1989) available for the region
(23–271 K). Since both heat capacities of ice and methane do not deviate substantially from
straight lines over the temperature region of interest, the conclusion may be drawn that Δ d H
and consequently the slope of ln(P) against 1/T should be continuous. This fact may be sup-
ported by the experimental evidence from the work of Majid et al. (1969) for cyclopropane
hydrates, where the sharp changes of slope of the hydrate equilibrium line were observed in
the hydrate structural transition region.
The absence of slope discontinuities in the entire subzero methane hydrates equilibrium
line suggested that there was no structural transition of the structure I hydrates to structure
II in the region of interest.

Xenon sI and xenon + neohexane sH hydrate experiments
Data in the literature for Xe hydrates
The second part of research on the low temperature apparatus was performed with a mix-
ture of hydrate formers. Pure xenon and xenon + neohexane were chosen as the structure I and
structure H hydrate formers, respectively. Pure xenon is only capable of forming sI hydrate.
Data for sI hydrate of xenon were published by Aaldijk (1971). One unpublished data point by
Dyadin et al. is at a much higher pressure. No previous phase equilibrium data was available
for sH hydrate of this two guest mixture. An investigation of a temperature - pressure phase
diagram of sH hydrate was the main purpose of this work.