Page 289 - Handbook Of Multiphase Flow Assurance

288 10. Research methods in flow assurance

Knight and coworkers have reported inhibition of ice growth by an antifreeze glycopep-
tide biomolecule which adsorbs in certain orientations on ice crystals (Knight et al., 1991,
1993). The ice crystallization rate in presence of such glycopeptides is up to five times greater
than in pure water (Harrison et al., 1987).
Bromley et al. (1993) has reported that diphosphonates caused morphology change in
barite crystals. L-leucine was documented to adsorb on glycine crystals and to inhibit their
growth (Li et al., 1994). Chen et al. (1994) has studied the adsorption of additives on chloroni-
trobenzene crystals. He found that the crystal morphology was modified. In general, it can be
stated that certain additives adsorb on growing crystals and block crystal growth.
The so-called “kinetic” inhibitors do not always inhibit hydrate formation completely, as
thermodynamic inhibitors do. However, kinetic inhibitors can delay the formation of hydrate
for a period longer than the residence time of the water phase in a pipeline. Kinetic inhibition
is currently applied in petroleum industry together with thermodynamic inhibitors to delay
hydrate formation in wells and in pipelines (Lederhos et al., 1996). Bloys and Lacey (1995)
reported successful KHI application to a production system with residence time of 120 h or
5 days.
Several successful kinetic inhibitors were found by an extensive screening of the com-
mercially available chemicals (Long et al., 1994; Lederhos, 1996). Over 1500 chemicals were
tested for their effectiveness of hydrate inhibition in a THF screening apparatus. If a chemical
inhibited hydrate growth in the THF screening apparatus, it was then tested in a high pres-
sure apparatus with natural gas. Delay time before visible hydrate formation was determined
as the effectiveness of hydrate inhibition was measured at 0 °C, which is 4.4 K lower than
the equilibrium temperature of THF hydrate melting at atmospheric pressure. The relative
performance of several inhibitors in terms of natural gas consumption versus time in a high
pressure apparatus is presented in Fig. 10.53. While the initial rate of hydrates formation is
high, the overall conversion rate is lowered by inhibitors. O

Generally, kinetic inhibitors are polymer molecules having an amide ( C N) linkage
in their side groups. Both poly-N-vinyl pyrrolidone (PVP) and poly-N-vinyl caprolactam
(PVCap) shown in Fig. 10.54 have rings of carbon atoms as their side groups. It was deter-
mined that the ring structure is not imperative to an effective inhibitor. A kinetic inhibitor
poly(N,N-diethyl acrylamide) (PNNDEAM) differs from PVP by position of the carbonyl
(CO) group and by the absence of a covalent bond between the two carbons opposite from
the nitrogen atom in the pyrrolidone ring. Hydrogen bonding ability is a necessary but not a
sufficient property of a kinetic inhibitor. Poly(vinyl alcohol) (PVA) has a hydrogen bonding
capability with the hydroxyl (OH) group; however, it is not a hydrate inhibitor.
One successful inhibitor is VC-713, a copolymer of about 30% vinyl-pyrrolidone, as in
PVP; 60% of vinyl-caprolactam, as in PVCap, and about 10% of dimethylaminoethylmeth-
acrylate (DMAEMA). The composition of this copolymer indicated the study of copolymer
of PVP and PVCap monomers in order to improve the kinetic inhibitor's performance. With
50% caprolactam +50% pyrrolidone this copolymer provided a larger hydrate formation
than PVCap or VC-713 alone, but with 75% caprolactam +25% pyrrolidone the copolymer
performed like PVCap (Figures 40, 41 in the Center for Hydrate Research Report) (Annual
Report, 1993).