Page 305 - Handbook Of Multiphase Flow Assurance

304 10. Research methods in flow assurance

Effect of adding kinetic hydrate inhibitors on the morphology of growing hydrate
THF + water + inhibitors solution without salt
The next step in studying hydrate crystal growth was to identify the effect of inhibitor
polymers on the morphology and the growth rate of THF hydrate crystals. The follow-
ing chemicals were tested: poly(vinylpyrrolidone), poly(vinyl-caprolactam) and a random
copolymer of vinylpyrrolidone, vinylcaprolactam, and dimethylaminoethylmethacrylate.
These compounds are referred to as PVP, PVCap, and VC-713 respectively. These chemicals
provide hydrate inhibition in both THF and natural gas systems, showing that the THF-
water system is a good model for structure II hydrate formation from natural gas (Lederhos
et al., 1996).
In this work concentrations of 0.5 wt% and 0.1 wt% were tested at temperatures of +1, +2,
and +3 °C. We studied the solutions of PVP, PVCap, or VC-713 of both high (89 K or higher)
and low (10 K or less) molecular weights. The studied polymeric inhibitor chemicals were
obtained from BASF. Solid samples of the investigated inhibitors were dissolved in deionized
water obtained from IBM Labs. If an inhibitor was available in an alcohol solution, the solvent
was removed by evacuation.
An unusual phenomenon was observed when we added kinetic inhibitors to the aqueous
THF solution. The shape of the THF hydrate crystal grown in the presence of these chemicals
changed from octahedral (3-D) to planar hexagonal (2-D) (Figs. 10.64 and 10.65). Faces of the
hexagonal plate were postulated to be {111} since the angles at which faces meet are same as
in an octahedral crystal. Another evidence supporting the hypothesis that the planar crystal
faces are {111} is the growth of a plane on an octahedral crystal grown without an inhibitor
and then moved into inhibited solution (Larsen et al., 1996, this work). The plane is parallel
to the {111} face of the octahedral crystal from which it grew (Fig. 10.66) which also indicates
a {111} plane.
The same effect was observed both at a 0.5 wt% concentration and at the lower concentra-
tion of 0.1 wt% of these chemicals in the test solution. All crystals grown in the presence of
hydrate inhibitors had visible defects. After hydrate crystals reached the sample tube walls,
hydrate also started to grow on the tube walls.
We hypothesize that the polymer chains of kinetic inhibitors adsorbed on the {111} surface
of the THF hydrate by hydrogen-bonding and blocked the further crystal growth. One possi-
ble explanation of the preference of the inhibitor adsorption towards the {111} face is that {111}
is the only surface available for adsorption since it's the slowest-growing face and other faces
have insufficient area for polymer adsorption on them. Another explanation may appeal to
the nature of {111} face. Inhibitor may adsorb stronger to {111} face compared to the other
faces because of a higher density of adsorption sites on that face.
We observed that the hydrate crystal growth was not completely inhibited by the kinetic
inhibitors at the concentrations listed above at the supercooling of 3.4 K, while at a lower
supercooling of 1.4 K the growth can be inhibited completely at a certain inhibitor concentra-
tion (Larsen et al., 1996; Makogon et al., 1997). The formed planar crystals grew in thickness,
as well as along the edge. This observation suggests that the adsorbed polymer allows for
hydrate growth beneath it. Otherwise, the polymer molecules must have been occluded by
the growing hydrate; however, the NMR study of hydrates by Ripmeester (1995) does not
confirm the occlusion phenomenon.