Page 233 - Handbook Of Multiphase Flow Assurance

232 10. Research methods in flow assurance

y
x

framework viewed along [001]

FIG. 10.10 Stereoscopic view of sH. Source: Meier, W. M., Olson, D. H. 1987. Atlas of Zeolite Structure Types. 2nd
revised ed. London: Butterworth.

by water. Small size allows these molecules to escape from the cavity. And in case the small
guest molecule prefers to stay inside, it will not provide sufficient support to the surrounding
water molecules and the cavity will collapse.
Hydrates may form from pure components as well as from the mixtures of hydrate formers.
Structure I and II hydrates may form with pure gases. Examples may be methane forming
structure I hydrate and nitrogen forming structure II hydrate. However, structures I and II
may be formed with mixtures of gases as well. Structure H hydrate must be formed with a
mixture of components. The variation in size of sH hydrate is so large that the hydrate cavi-
ties cannot be stabilized by guest molecules of one size.
Not every cavity is occupied in hydrate by a guest molecules. In order for the hydrate to
be stable the occupancy of the hydrate lattice must be high. For this requirement to be met it
is necessary to have sufficient availability of the guest molecules of appropriate size. In case
12
of structure II hydrate there should be enough methane molecules to fit into the small 5
12 4
cavities and propane molecules to stabilize the large 5 6 cavities.
Gas hydrates may change their structure depending on thermodynamic conditions and com-
position of hydrate former. The van der Waals and Platteeuw (1959) statistical thermodynamics
model is most frequently used in fitting and in predicting the equilibrium conditions of hydrates
formation. A recent use of the model by Lundgaard and Mollerup (1992) suggested an unusual
prediction of the phase diagrams of methane hydrates, obtained via minimization of the Gibbs free
energy of the system. One of the predictions of Lundgaard and Mollerup was that a slight mismea-
surement of the unit crystal cubic side (by as little as 0.002 nm in 1.2 nm) could cause a structural
transition (I to II) on the three phase (I-H-V) hydrate equilibrium line at a temperature of 170 K.
Such a transition would not be unique because cyclopropane and trimethylene oxide have
the ability to form either structure I or II hydrates, depending on thermodynamic conditions.
The simple hydrates of cyclopropane were shown to undergo structural transition in the tem-
perature range of 257.1–274.6 K based on data by Hafemann and Miller (1969) and Majid et al.
(1969). Hydrates of trimethylene oxide undergo phase transition between 252.4 and 260.1 K as
determined by Hawkins and Davidson (1966).
Several other works (e.g., Holder and Hand (1982), Adisasmito and Sloan Jr. (1992), etc.)
provided experimental evidence for structure I - structure II transition for hydrates of natural
gas mixtures at temperatures above the ice point. However, for gas mixtures, the phase tran-
sition occurs as a principal function of gas composition.