Page 275 - Handbook Of Multiphase Flow Assurance

274 10. Research methods in flow assurance

Normalized number
of hydrogen bonded rings
per molecule
0.3

0.25

0.2

0.15

0.1

0.05

0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Number of water molecules in hydrogen bonded ring.
FIG. 10.45 Distribution of hydrogen bonded polygons in water at 298 K dominated by pentagons and hexagons.

Effect of macromolecules on the structure of water
Several polymers are known to be gas hydrate formation inhibitors. When certain poly-
mers are introduced into water, hydrate formation is deferred for some time on the scale of
minutes to hours. The testing of the hypotheses for the mechanism of inhibition is a topic of
this section.
The first hypothesis was the docking of polymer chains on the growth sites of hydrate.
This presumes the irreversible adsorption as in the case of winter flounder polypeptide.
The other mechanism can be sought as change of the water structure by the polymer away
from that favoring hydrate formation (i.e., decreasing the number of 5- and 6-membered rings
in water). This hypothesis can fit into the kinetic nature of inhibition since water molecules
can reorganize with time back to an abundance of 5- and 6-membered polygons. A number of
simulations was run to test the second hypothesis. The main outcome of this part of research
is that the relative computer simulation performance of inhibitors is in accord with the real
experiments. In addition a new inhibitor was designed via computer which outperformed
PVCap by nearly a factor of two.
Effect of VC-713 Inhibitor
A 9 repeat unit, 28 monomer chain of VC-713 polymer composed of 596 atoms was solvated
in 827 SPC water molecules. Initial conformation of the polymer was the same as shown in
Fig. 10.32. Atoms of the polymer did not have charges on them. Charges on the atoms of water