Page 309 - Handbook Of Multiphase Flow Assurance

308 10. Research methods in flow assurance

have pronounced effects on water hydrogen bonding (Makogon, 1994). The plate-like habit
was not found at chemical concentrations of 0.5 wt% and a supercooling of 3.4 K.
It follows from these results that the inhibitors can adsorb on {111} surfaces of hydrate.
This is a fundamental issue for understanding hydrate growth inhibition at the molecular
scale. Preferential adsorption of kinetic inhibitor on hydrate {111} faces was recently proven
experimentally (Larsen, 1996).

Effect of NaCl salt on THF hydrates
A set of experiments was performed with NaCl in solution at 1 °C (3.4 K supercooling).
NaCl is a known thermodynamic hydrate inhibitor (Sloan, 1990). The presence of NaCl in wa-
ter solution improves kinetic inhibitors' performance (Long, 1994). In present work it was also
determined that hydrate growth can be stopped completely if salt is added to the solution.
The synergism of salt with kinetic inhibitors may be explained using Langmiur adsorp-
tion isotherm. It was hypothesized (Long et al., 1994) that hydrate growth involves adsorp-
tion of guest molecules onto labile hydrate cavities. The rate of attachment to the surface
should be proportional to a driving force times an area. The driving force is the concentration
in the fluid, and the area is the amount of bare surface. If the inhibition is performed only
by the polymer adsorbed on hydrate, the rate of coverage with guest molecules at pseudo-
equilibrium would be equal to the rate of desorption from the surface, (if desorption does
occur for all polymer segments).
Desorption = Adsorption

k Guest Θ Guest + k Poly Θ Poly = k′ Guest C (1 − Θ Guest − Θ Poly) + k′ Polly C Poly ( 1− Θ Guest − Θ Poly) ,
Guest
where
θ i —fraction of the surface covered with species i,
k′ i , k i —pseudo-equilibrium adsorption and desorption rate constants.
Rearrangement leads to the Langmiur isotherm expression:

K C
Θ = Guest Guest ,
Guest
1 + K Guest C Guest + K Poly C Poly
where
k′
K = i .
i
k Guest
If the inhibition is performed by the kinetic inhibitor adsorbed on hydrate and salt ions
with shells of water molecules blocking some adsorption sites, the Langmiur isotherm ex-
pression would look like:

K C
Θ Guest = Guest Guest .
1 + K Guest C Guest + K Poly C Poly + K Salt C Salt
An extra term in the denominator decreases the coverage of the hydrate surface with guest.
Results of investigating the hydrate growth habits with and without salt are summarized
in Table 10.13.