Kinetic inhibitors

A kinetic inhibitor is a polymeric chemical that, when added to a production stream, will not change the hydrate formation temperature but will delay the growth of hydrate crystals. These chemicals are polymeric... 

The use of kinetic inhibitors and/or anti-agglomcrators in actual fieid operations is a new and evolving technology. These are various formulations of chemicals that can be used in a mixture of one or more kinetic inhibitors and/or anti-agglomerators. At the current time, to get an optimum mixture for a specific application it is necessary to set up a controlled bench test using the actual fluids to be inhibited and determine the resulting equilibrium phase line. As the mixture of chemicals is changed, a family of equilibrium phase lines will develop. This will result m an initial determination of a near optimum mixture of chemicals. 

Kinetic inhibitors for hydrate formation may also be effective in preventing scale deposition [1627]. This may be understood in terms of stereospecific and nonspecific mechanisms of scale inhibition. 

The usual practice for avoiding the plugging of production facilities by hydrates is to add thermodynamic inhibitors, such as methanol or glycol. A newer concept is the injection of low-dosage additives either kinetic inhibitors, which delay nucleation or prevent the growth of hydrate crystals, or hydrate dispersants, which prevent the agglomeration of hydrate particles and allow them to be transported within the flow [880,1387]. Hydrate control is discussed extensively in Chapter 13. Classes of hydrate control agents are shown in Table 11-9, and additives are shown in Table 11-10. 

J. S. Pic, J. M. Herri, and M. Coumil. Experimental influence of kinetic inhibitors on methane hydrate particle size distribution during batch crystallization in water. Can J Chem Eng, 79(3) 374-383, June 2001. 

Research in this field is ongoing aiming to understand the mechanism of action of kinetic inhibitors. Lee and Englezos (2005) showed that inclusion of polyethylene oxide (PEO) to a kinetic inhibitor solution was found to enhance by an order of magnitude the performance of the hydrate inhibitor. Binding of inhibitor molecules to the surface of hydrate crystals was considered to be the key aspect of the mechanism of kinetic inhibition (Anderson et al.,... 

Kinetic inhibitors exhibit unusual effects on hydrate formation with implications to processing (Lee and Englezos, 2006). Gas hydrate formation experiments were conducted... 

Lee, J.D. Englezos, P. (2005) Enhancement of the Performance of Gas Hydrate Kinetic Inhibitors with Polyethylene Oxide. Chem Eng Sci, 60, 5323-5330. 

Effect of Aromatics on Formation of Coke and Carbon. Conventional liquid fuels contain widely differing levels of aromatics gasoline usually contains more than diesel. The studies show that these compounds cause more rapid deactivation than linear alkanes alone. A related result is that the steady state conversion of diesel fractions is also reduced in the presence of aromatics—i.e., these compounds act as kinetic inhibitors, limiting the production of H2. 

Lederhos, J., The Transferability of Hydrate Kinetic Inhibitor Results between Bench Scale Apparatuses and a Pilot Scale Flow Loop, Ph.D. Thesis, Colorado School of Mines, Golden, CO (1996). 

The inhibition of three-phase hydrate formation is discussed in Section 4.4. These predictions enable answers to such questions as, How much methanol (or other inhibitor) is required in the free water phase to prevent hydrates at the pressures and temperatures of operation Classical empirical techniques such as that of Hammerschmidt (1934) are suitable for hand calculation and provide a qualitative understanding of inhibitor effects. It should be noted that only thermodynamic inhibitors are considered here. The new low-dosage hydrate inhibitors [LDHIs, such as kinetic inhibitors (KIs) or antiagglomerants (AAs)] do not significantly affect the thermodynamics but the kinetics of hydrate formation LDHIs are considered in Chapter 8. 

Itoh et al. (1996) used MD to explain the CO2 bending and stretching peaks in Raman spectra. Recently Carver et al. (1995), Kvamme et al. (1996), Makogon (1997), and Anderson (2005) used MD to model hydrate kinetic inhibitors interactions with the crystal surface. 

An alternative that is less resource-intensive than the flow loop is the flow wheel apparatus (Bakkeng and Fredriksen, 1994 Lippmann et al., 1994) shown in Figure 6.4b. The wheel (torus) is nominally a 2-5 in. (5.1-12.7 cm) pipe, 2 m in diameter that rotates at 0.3-5.0 m/s while filled with gas and less than 50 vol% liquid. Conceptually, the wheel is spun past the gas and liquid rather than the reverse. Therefore, the flow wheel apparatus does not require circulating devices such as pumps or compressors. Hydrate formation is deduced visually, or by a sharp increase in torque required to turn the wheel. Urdahl et al. (1995) and Lund et al. (1996) report good field transferability from results obtained with this apparatus. Pilot flow loops and flow wheels have been also used to simulate shut-in/start-up conditions (12 h stagnant period) and to test kinetic inhibitors (e.g., Palermo and Goodwin, 2000 Rasch et al., 2002). 

Small angle neutron scattering instruments are specifically designed to examine disordered materials, such as to determine hydration structures during hydrate formation (Koh et al., 2000 Buchanan et al., 2005 Thompson et al., 2006), or to study kinetic inhibitor adsorption onto a hydrate surface (Hutter et al., 2000 King et al., 2000). 

The following sections present three examples of kinetic phenomena (1) kinetic inhibitors, (2) antiagglomerants (AAs), and (3) hydrate plug remediation. These kinetic phenomena were determined by field and laboratory observations. They also point to the need for a comprehensive kinetics theory, from which hydrate nucleation and growth can be predicted for industrial utility. 

FIGURE 8.10 Hydrate kinetic inhibitors. Unless indicated, every angle is the location of a carbon atom and the appropriate number of hydrogens. 

Examples of kinetic inhibitor chemicals are shown in Figure 8.10. In the figure, each inhibitor is shown with a polyethylene backbone, from which a pendant group (typically a ring compound with an amide [—N—C=0] linkage) is suspended. There are several types of kinetic inhibitors, and due to proprietary... 

The definitive hydrate kinetic inhibition mechanism is not yet available. Some work suggests that the mechanism is to prevent hydrate nucleation (Kelland, 2006). However, a significant amount of evidence suggests that hydrate kinetic inhibitors inhibit the growth (Larsen et al., 1996). However, this apparent conflict is due to the definition of the size at which crystal nucleation stops and growth begins. To resolve this confusion, one may consider growth to occur after the critical nucleus size is achieved. 

It should be noted here that, while PVP was one of the first kinetic inhibitors discovered, it is one of the weakest kinetic inhibitors available. 

In his review of LDHIs, Kelland shows that kinetic inhibitors are well-established tools for hydrate prevention, with the following three points ... 

Low molecular weight PVCap-based products with added synergists were the best kinetic inhibitors for structure II hydrates on the market in 2005, and these inhibitors can provide 48 h of inhibition at a subcooling of 13°C. 

In 2005, 40-50 applications of kinetic inhibitors operated worldwide, with the largest applications in the North Sea and Qatar Applications. 

Table 8.2 summarizes the chronological development of AAs. As of April 1, 2004, AAs were used in 17 gas and oil fields in the Gulf of Mexico, with prospects for rapid expansion. However, as of this writing, kinetic inhibitors are predominantly used in LDHI work. 

The industrial flow assurance paradigm is shifting from avoidance, enabled by thermodynamic inhibition, to risk management, enabled by application of kinetics. Examples of time-dependent flow assurance phenomena are kinetic inhibitors, AAs, plug dissociation, and electrical heating of pipelines for plug dissociation. Research support will move from thermodynamics, which is currently acceptably accurate for engineering applications, to time-dependent kinetics. 

Long, J., Lederhos, J., Sum, A., Christiansen, R., Sloan, E.D., Kinetic Inhibitors of Natural Gas Hydrates, in Proc. 73rd Annual Convention of Gas Processors Association, New Orleans, LA, March 7-9, p. 85 (1994). 

---