Page 312 - Handbook Of Multiphase Flow Assurance

Experimental study of hydrate crystal growth 311

If the diffusion of nutrients to the crystal is hindered by the inhibitor adsorbed to the crys-
tal face, then the diffusion-controlled growth occurs. In such case any protrusion of the crystal
surface may encounter more nutrient. Growth at the end of the protrusion will be favored
over growth on flat crystal face. It was observed that in case of thin planar crystal growth in
presence of PVCap or VC-713 the growth rate at the edge of the plane was faster than at the
crystal face by at least an order of magnitude.
Change of crystal habit from octahedral to planar at non-stoichiometric hydrate melt
compositions described above (Larsen et al., 1996) can also be attributed to a transition to
a diffusion- controlled growth. As the concentration of either nutrient (water or THF) drops
near the growing surface, a planar crystal growth becomes preferred.
There are three common types of crystal growth mechanisms illustrated in Fig. 10.69:
(1) Screw-dislocation. This is growth around a defect in a spiral direction. Such growth
of crystals results in acicular, or needle-like growth. It occurs at very low degrees
of supersaturation. Acicular crystal growth is seldom observed experimentally.
Murowchick and Barnes (1987) have observed acicular growth of pyrite only once in a
set of 70 experiments. We observed such growth only once in all experiments (Fig. 10.70)
with aged PVCap solution.
(2) Layer-by-layer. This type of growth occurs at moderate degrees of supersaturation.
Such mechanism involves two-dimensional nucleation and lateral spreading from these
nuclei on surface. Growth steps are visible on the crystal surface in Figs. 10.60 and 10.64.
The large steps indicate a high nucleation rate causing the spreading layers to pile up on
one another.
(3) Continuous. In this type of growth building blocks attach to any crystal surface
when the nucleation rate is extremely high. This occurs at very high degrees of
supersaturation and when heat of crystallization is low. A product crystal does not show
any crystal faces and looks like a boulder (Myerson, 1993).
It was experimentally observed at CSM Hydrate Center that polymers of lower molecular
weight (MW) have better performance than those of higher MW. Diffusivity of lower MW
polymers is higher than that of polymers with higher MW. Lower MW inhibitors can reach
hydrate surface and block its growth faster than higher MW ones.

(A) (B)
FIG. 10.69 Mechanisms of crystal growth.