Page 298 - Handbook Of Multiphase Flow Assurance

Experimental and computer study of the effect of kinetic inhibitors on clathrate hydrates 297

Adsorption of PVP of different chain lengths on sI hydrate was modeled using MC module
2
of the commercial software Cerius (Carver et al., 1996). It was found that the adsorption of
rigid PVP chains of 1, 2, 4, and 8 mers on hydrate was energetically favorable. Formation of
loops in adsorbed state was reported for a rigid PVP octamer. Monomers were found to ad-
sorb on an edge of a partially completed large cavity by forming two hydrogen bonds.
2
A similar work was performed at CSM using the Cerius software to form monomers of
inhibitors on hydrates. This work gave us preliminary results about locations of inhibitor
monomers adsorption on sII hydrate. In our work the monomers were found to adsorb both
inside the hydrate cavities and on their periphery. The monomers adsorbed in the partial hy-
drate cavity had their carbonyl (CO) oxygens pointed at hydrogens on the hydrate surface
indicating hydrogen bonding.
Koh et al. (1996)) has presented time-resolved NMR measurements for hydrate structure in
presence of PVP and tyrosine. No definitive conclusion was made about where PVP adsorbs
on sII hydrate.
Kvamme and colleagues have presented their results on molecular dynamics simulation of
PVP in water and water/hydrate systems on both International hydrate conferences. The first
work (Kvamme, 1994) indicated that a PVP monomer hydrogen-bonded to water in solution
via its carbonyl group gradually changes its bond towards the water in hydrate. The contin-
ued work (Kvamme et al., 1996) simulated behavior of PVP as side group ring without the
backbone part. The inhibitor was observed to orient perpendicular to the hydrate surface.
As a general conclusion, various characteristics of liquid water, ice and clathrate hydrates
obtained through modeling are summarized in Table 10.12. At present, potential models need
improvement in order to be able to truly simulate gas hydrate behavior.
Over the last 6 years all commercially available chemicals have been tested, and there
is a need to predict new chemicals which can be synthesized and then tested in a screen-
ing apparatus. The goal of this work is the design of kinetic inhibitors using computer
modeling. Therefore the mechanism of kinetic inhibition of gas hydrates formation is of
critical interest. No experimental information was available to-date about the mechanism of
kinetic inhibition. Three hypotheses for the kinetic inhibition mechanism were developed at
CSM (Long, 1994):
(1) A classical inhibition mechanism through adsorption of an inhibitor molecule on the
growing surface of a crystal and preventing hydrate growth.
(2) Modification of the structure of water in vicinity of an inhibitor to make it unfavorable
for hydrate formation. This hypothesis originated from CSM (Makogon, 1994).
(3) A mass transfer limitation caused by polymeric chains preventing agglomeration of
hydrate.

Research objectives
The present work analyzed experimental and computer modeling evidence for the adsorp-
tion hypothesis. There were eight research objectives for this work:
1. Design and use a new apparatus to determine preferred growth planes of sII hydrate,
2. measure the effects of kinetic inhibitors and/or NaCl at different concentrations on
hydrate growth and on the preferred direction of a sII hydrate crystal growth,