Page 235 - Handbook Of Multiphase Flow Assurance

234 10. Research methods in flow assurance

HH pair correlation function doesn't change much from that for the pure water. This work
also showed the experimental evidence that water molecules form a disordered hydrogen
bonded cage around the methanol molecule.
The water structure can be described not only by the pair correlation function but also by the
structure of hydrogen bonded network in water. No difference can be seen for the pair correlation
function of pure water and water + hydrate inhibitor solution. The changes in hydrogen bonded
network of aqueous inhibitor solution were significant compared to pure water. Formation of the
hydrogen bonded cage around the inhibitor monomers was also noticed in computer simulation.
Phase equilibria data for hydrate formation with inhibitors is available in the literature.
Sloan (1990) reviewed the data for pure hydrate forming natural gases and their mixtures.
There has been some contradiction in the literature on the effect of hydrate equilibrium with
low concentrations of alcohol. Makogon (1981, p. 134) and Berecz and Balla-Achs (1983, p. 102)
reported that at concentrations below approximately 5 wt% of methanol in water the onset of
hydrate formation can occur at a higher temperature, thus reducing the subcooling required to
start hydrate formation. A hypothesis was suggested by Makogon (1981) that this may happen
due to inclusion of the methyl CH 3 radical in voids in the structure of water.
It was also reported later (Svartas and Fadnes, 1992) that methanol inhibited the hydrate
formation over the whole range of concentrations. Only a few data points in this work, which
indicated the opposite effect, were related to an experimental error.
Another interesting concept discussed in the literature is whether methanol molecules can
or cannot participate in the hydrate structure. NMR and dielectric study of ethylene oxide
and tetrahydrofuran hydrates was performed (Davidson et al., 1981). The results show no
sign of enclathration of methanol.
The opposite was suggested by computer studies performed on water-methane-methanol
mixtures (Wallqvist, 1992) at 270 K. He made a simulation of a single unit cell of sI methane
hydrate with a varying number of methane molecules substituted by methanol molecules.
He reported that small amounts of methanol can be incorporated into the hydrate structure.
A 4 wt% methanol solution hydrate was reported to be stable, whereas a 7 wt% solution hy-
drate melted. Experimental evidence for the formation of a solvation shell of water molecules
around methanol molecules is drawn from a neutron diffraction study (Soper and Finney,
1993) of a 1:9 M ratio methanol-water mixture.

Kinetics of hydrate formation

Studying the kinetics of hydrate formation allows one to determine two attributes of hy-
drate formation. One is how soon hydrate will start forming (induction time) since the system
was placed in appropriate thermodynamic conditions. The other attribute is the growth rate
at which liquid water or ice will be converted into a solid hydrate.
Interest in this area of hydrate research has previously been purely academic. Today indus-
try is seeking the new chemicals which allow operation of a gas-water system at conditions
where hydrate would normally form, so that it stays in a metastable state without aggregat-
ing to a large hydrate mass.
Experimental data for kinetics of hydrate formation are available in the literature for tem-
peratures above and below the ice point. Falabella (1975) studied the formation of hydrates of
different gases and mixtures of gases at low sub-zero temperatures. The work on kinetics of
hydrate formation have been reviewed (Sloan, 1990).