Page 300 - Handbook Of Multiphase Flow Assurance

Experimental study of hydrate crystal growth 299

Experimental study of hydrate crystal growth

Morphology of hydrate crystals
The main objective of this part of research was to study the shape of hydrate crystal, the
morphology of crystal faces, and the growth rate of hydrate crystals, with respect to the fol-
lowing variables:

(1) temperature;
(2) concentration of added kinetic inhibitors;
(3) molecular weight of added kinetic inhibitor;
(4) concentration of NaCl salt added (with and without added kinetic inhibitor).
An elementary principle of crystal growth is that the slowest growing crystal faces are pre-
served, and the faces which grow fast grow out of existence as illustrated in Fig. 10.57. Thus
a control experiment of growing a single crystal from pure hydrate melt gives information
about the orientations with the slowest growth kinetics. Additives may produce new slowest-
growing faces, in which case one discovers the surface orientations to which they adsorb.
This technique has been used successfully for the antifreezes evolved by polar fish to prevent
ice crystal growth in their blood (Harrison et al., 1987; Knight et al., 1991; Knight et al., 1993).
A visual apparatus for observation of the THF hydrate growth was constructed. The appa-
ratus (Fig. 10.58) consisted of a transparent Plexiglas cooling jacket with inserted glass sample
tube containing a glass pipette. The test solution was placed inside a sample tube and cooled
below the hydrate melting point. Then the glass pipette was placed into the solution, and a
copper wire cooled with liquid nitrogen or with ice water was inserted into the pipette in
order to initiate hydrate formation inside the pipette. The pipette served as a conduit for the
growing crystal. The hydrate crystal grew to the edge of the pipette and a small area of the
crystal became exposed to the solution in the test tube. Hydrate started to grow as a single
crystal on the tip of the pipette in the test solution inside the test tube.
A study of the effect of temperature on crystal growth was performed. From equilibrium
considerations, the temperature at the growing crystal surface equals to the equilibrium
temperature of hydrate decomposition. The greater the temperature difference between the
crystal surface and the bulk solution, named supercooling, the greater the driving force for
hydrate nucleation, and the faster the crystal growth. The heat of crystal nucleation Δ d H is
dissipated into the bulk solution via convective flow of solution near the crystal. Since there
is no mass transfer change in growth from THF hydrate melt, the growth is heat-transfer
limited. Crystal growth experiments held temperature, pressure, and composition constant.

FIG. 10.57 Disappearance of a fast-growing crystal face (long arrow).