Page 337 - Handbook Of Multiphase Flow Assurance

336 10. Research methods in flow assurance

The curves in Fig. 10.86 are only meaningful up to the temperature of about 200–250 K
above which hydrate would normally melt. The empty hydrate lattice was artificially fixed
in place and could not melt. It was determined that the SPC ice loses its structure (melts) at
a temperature of 200 K (Karim et al., 1990). The difference in chemical potential between ice
and an empty sI hydrate lattice is only 0.31 kcal/mol (Tse et al., 1986).
For sII hydrate the difference is even less at 0.25 kcal/mol, which is less than one hydrogen
bond's strength.
There are several MD simulation estimates of hydrate stability limit ranging from 205 K
(Tse et al., 1983b) to 330 K (Forrisdahl et al., 1996). Tse and coworkers have observed in their
MD study of sI hydrate using SPC model that translational frequency spectra for empty hy-
drate lattice at 110 K was different from that at 205 K indicating a possible phase transition.
A recent study by Forrisdahl et al. (1996) of hydrate stability with methane guests in an NVT
MD used SPC model for water. The authors indicate that the hydrate stability limit of 330 K
is an overestimate of an experimental value. Guest molecules significantly stabilize empty
hydrate lattice (Rodger, 1991; Tse, 1994).
In the present work we assume that the SPC empty hydrate lattice melting temperature is
same as for SPC ice, i.e., empty hydrate lattice would melt at about 200 K and the results for
higher temperatures would be for unstable hydrate.
The shape of curves in Fig. 10.86 is attributable to the nature of methane interaction with
the hydrate surface and/or the adsorbed polymer. Without polymer on the surface at low
temperature limit methane condenses and doesn't move out of a hydrate cavity. The number
of cycles which methane spent outside hydrate cavities increased with temperature.
In presence of a polymer at 100 K temperature methane condensed on the polymer and
spent less time in hydrate cavities. This results in low adsorption of guest into open cavities.
At 150 K there is a peak in adsorption curves. Methane which mostly adsorbed on polymer
at 100 K is now able to desorb from polymer and reach the hydrate surface. The temperature
is still sufficiently low for methane's thermal motion to overcome the attraction of hydrate
cavities. Methane mostly remains adsorbed in hydrate cavities. This results in a peak in ad-
sorption curves.
At 200 K methane doesn't adsorb in cavities as often because of a higher temperature and
its higher thermal motion. At temperatures above 200 K methane adsorbs less on hydrate and
moves in and out of hydrate cavities. The fraction of methane adsorption on hydrate is higher
with polymers present than on a bare surface because polymer creates additional attraction
for methane towards the hydrate.

Using the computer to design inhibitors
The properties which define a good inhibitor are:

(1) Blocking of guest molecules adsorption on hydrate. This requires (a) relatively large
steric size of a side group (8–12 atoms excluding hydrogens and backbone atoms)
and (b) presence of hydrophobic part in side group for attracting hydrocarbon guest
molecules.
(2) Strong adsorption to surface. In all simulations inhibitors adsorbed stronger than
non-inhibitors. This requires a polar part in side group (like CO) sufficient to make
adsorption and hydrogen bonding of the hydrophobic side group on hydrate favorable.