Page 281 - Handbook Of Multiphase Flow Assurance

280 10. Research methods in flow assurance

Summary of the experimental and computer model work
The first part of the work was experimental. A question about the possibility of the phase
transition of methane hydrate was solved negatively. The second part was also experi-
mental. The study of xenon sI and xenon + neohexane sH hydrates was done. A quintuple
point sH-sI-Lw-Lh-V was shown to occur in the temperature interval around 281.5 K in
xenon + neohexane system.
The last part of the work was performed on the IBM RS-6000 model 550 computer with
128 Mb RAM using the SYBYL 6.01 software. Docking of macromolecules on hydrate and ice
surfaces showed that Winter Flounder polypeptide, PVP and PVCap chains may adsorb on
ice and hydrates, whereas VC-713 may not. SPC water model was adequately representing
the structural and thermodynamic properties of the real water. Study of the influence of gas
hydrate inhibitors on the structural properties of water allowed to propose a new kinetic hy-
drate inhibitor. A hypothetical explanation of the hydrate inhibition mechanism was stated.

Experimental and computer study of the effect of kinetic inhibitors on
clathrate hydrates

This part of research work consists of an experimental and theoretical investigation of
interaction between clathrate hydrates and kinetic inhibitors. There is experimental and com-
putational evidence in support of the hypothesis that kinetic inhibitors act by adsorbing with
their side groups inside or near the incomplete large cavities on a hydrate surface and block
adsorption of guest molecules into hydrate cavities.
Experiments showed that single crystals of THF hydrate grew as octahedra with {111} crys-
tallographic faces. A change in morphology of a THF hydrate crystal was observed upon ad-
dition of a hydrate inhibitor whereupon an octahedral crystal became planar with {111} faces
due to inhibitor adsorption on hydrate.
Computer modeling of inhibitor adsorption on the hydrate surface showed that polymers
strongly adsorb in a flat (train) conformation at the periphery of large open hydrate cavities.
Adsorbed polymers inhibit adsorption of guest molecules at the growing hydrate surface.
The computer was used to design more effective hydrate inhibitors.
The pressure and temperature at which hydrate forms determine the ratio of guest mol-
ecules to water molecules. Generally, hydrate formation is favored at high pressures and at
low temperatures, and the water-guest ratio is close to 6.
The first documented formation of clathrate hydrate dates about 200 years back when
SO 2 hydrates (Priestley, 1790) and Cl 2 hydrates (Davy, 1811) were formed. However, only the
growth of gas and oil industry caused hydrates to gain industrial importance when they were
found to plug natural gas pipelines (Hammerschmidt, 1934).
The molecules which form clathrates range in size from hydrogen (Vos et al., 1993) to eth-
ylcyclohexane (Thomas and Behar, 1994). Sizes of these molecules are estimated as 3.0 and
9.7 Å, respectively. The highest pressures for which hydrate formation data were reported
are 2 GPa for nitrogen (van Hinsberg et al., 1993), 30 GPa for hydrogen (Vos et al., 1993), and
1.5 GPa for argon and krypton (Dyadin et al., 1996).
Currently, the main task at hand is to understand the formation of gas hydrates and to
be able to prevent their formation and promote their decomposition in industrial facilities.