Hydrate nucleation

Hydrate nucleation is the process during which small clusters of water and gas (hydrate nuclei) grow and disperse in an attempt to achieve critical size for continued growth. The nucleation step is a microscopic phenomenon involving tens to thousands of molecules (Mullin, 1993, p. 173) and is difficult to observe experimentally. Current hypotheses for hydrate nucleation are based upon the better-known phenomena of water freezing, the dissolution of hydrocarbons in water, and computer simulations of both phenomena. Evidence from experiments shows that nucleation is a statistically probable (not deterministically certain see Section 3.1.3) process. 

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Knowledge concerning the mechanism of hydrates formation is important in designing inhibitor systems for hydrates. The process of formation is believed to occur in two steps. The first step is a nucleation step and the second step is a growth reaction of the nucleus. Experimental results of nucleation are difficult to reproduce. Therefore, it is assumed that stochastic models would be useful in the mechanism of formation. Hydrate nucleation is an intrinsically stochastic process that involves the formation and growth of gas-water clusters to critical-sized, stable hydrate nuclei. The hydrate growth process involves the growth of stable hydrate nuclei as solid hydrates [129]. 

Nucleation and growth of gas hydrate crystals have been investigated with optical methods under different pressures and temperatures. The particle sizes measured during gas hydrate nucleation ranged from 2 to 80 imi [1334,1335]. The nucleation process is nondeterministic, because of a probabilistic element within the nucleation mechanism [1393]. 

Kvamme, B. Graue, A. Aspenes, E. Kuznetsova, T. Granasy, L. Toth, G. Pusztai, T. Tegze, G. (2004). Kinetics of solid hydrate formation by carbon dioxide Phase field theory of hydrate nucleation and magnetic resonance imaging. Phys. Chem. Chem. Phys., 6, 2327-2334. 

Ohmura, R. Ogawa, M. Yasuoka, K. Mori, Y.H. (2003b). Statistical study of clathrate-hydrate nucleation in a water/hydrochlorofluorocarbon system Search for the nature of the "memory effect". J. Phys. Chem. B, 107 (22), 5289-5293. 

Hydrate nucleation (Section 3.1), which is a stochastic process,... 

Two questions of hydrate time-dependent phenomena are essential to both industry and researcher (1) When will hydrates nucleate (2) Once nucleated, how rapidly will hydrates grow or dissociate ... 

As an example of hydrate nucleation and growth, consider the gas consumption versus time trace in Figure 3. la for an agitated system operated at constant pressure and temperature. An autoclave cell (e.g., 300 cm3) containing water (e.g., 150 cm3) is pressurized with gas and brought to hydrate formation (P, T) conditions. The gas is added from a reservoir to maintain constant pressure as hydrates form with time. The rate of consumption of gas is the hydrate formation rate that can be controlled by kinetics, or heat or mass transfer. 

The induction time is marked as 1 and includes the time taken for crystal nuclei to form which are not visible to macroscopic probes. The induction time is defined in practice as the time elapsed until the appearance of a detectable volume of hydrate phase or, equivalently, until the consumption of a detectable number of moles of hydrate former gas. The induction time is often also termed the hydrate nucleation or lag time (Section 3.1). (The induction or lag time is the time taken for hydrates to be detected macroscopically, after nucleation and onset of growth have occurred, whereas nucleation occurs on too small a size scale to be detected. Therefore, the term nucleation time will not be used in this context. Instead, the term induction time or induction period will be used. The induction time is most likely to be dominated by the nucleation period, but also includes growth up to the point at which hydrates are first detected.)... 

Nucleation thermodynamics were published by Ginns in 1928 based upon his work at the end of the last century. Volmer and Weber (1926) indicated that the growth and decay of clusters of molecules played a major role in nucleation kinetics. The most recent reviews of hydrate nucleation are by Kashchiev and Firoozabdi (2002a,b). 

In order to achieve some understanding of the nucleation of hydrate crystals from supercooled water + gas systems, it is useful to briefly review the key properties of supercooled water (Section 3.1.1.1), hydrocarbon solubility in water (Section 3.1.1.2), and basic nucleation theory of ice, which can be applied to hydrates (since hydrate nucleation kinetics may be considered analogous, to some extent, to that of ice Section 3.1.1.3). The three subsections of 3.1.1 (i.e., supercooled water, solubility of gas in water, and nucleation) are integral parts of conceptual pictures of nucleation detailed in Section 3.1.2. 

Makogon (1974) was the first to incorporate the above concepts into a hydrate nucleation mechanism, indicating that water molecules cluster with a decrease in temperature. 

Hydrate nucleation and growth may have direct analogies in crystallization processes such as the precipitation of salt from solution. Metastability in salt crystallization was hypothesized to occur through supersaturation by Ostwald (1900). (A supersaturated solution is one in which the liquid [solvent] contains more dissolved solute than can be ordinarily accommodated at that temperature the greater the degree of supersaturation, the greater number of crystal nuclei that will form in solution.) Miers and Isaac (1907) experimentally proved metastability and postulated that for each solute-solvent pair, a concentration-temperature relationship exists that defines the metastable limit, formally called the thermodynamic spinodal. 

To determine the relationship between hydrate nucleation (requiring three phases) and the more usual type (two-phase nucleation) consider the theory of homogeneous and heterogeneous nucleation in crystallization, as reviewed by Mullin (1993, p. 172) and Kashchiev and Firoozabadi (2002b), from which much of the below discussion has been excerpted. 

The kinetics of nucleation of one-component gas hydrates in aqueous solution have been analyzed by Kashchiev and Firoozabadi (2002b). Expressions were derived for the stationary rate of hydrate nucleation,./, for heterogeneous nucleation at the solution-gas interface or on solid substrates, and also for the special case of homogeneous nucleation. Kashchiev and Firoozabadi s work on the kinetics of hydrate nucleation provides a detailed examination of the mechanisms and kinetic expressions for hydrate nucleation, which are based on classical nucleation theory. Kashchiev and Firoozabadi s (2002b) work is only briefly summarized here, and for more details the reader is referred to the original references. 

The authors further note that although visual observations have shown that hydrate crystallizes at the solution-gas interface, this may also be because of nucleation and subsequent growth within a thin solution layer adjacent to the solution-gas interface. For kinetic reasons, the supersaturation in the thin solution layer can be locally high, and therefore hydrate nucleation and subsequent growth in this layer would in fact be more probable than in the bulk of the solution. 

A hydrate nucleating agent (precipitated amorphous silica) and a quiescent surface inhibitor (sodium dodecyl sulfate) were used in an attempt to initiate hydrates in the bulk phase. While the induction time (for detectable hydrate formation) was not predictable, in every case hydrate was initiated at a surface—usually at the vapor-water interface, but infrequently along the sides of the sapphire tube in the gas phase, and at the metal end-plate below the liquid phase. 

Conceptual Picture of Hydrate Nucleation at the Molecular Level... 

FIGURE 3.12 Methane-methane radial distribution functions calculated from successive 0.9 ns portions of the simulation, indicating ordering of the methane molecules during hydrate nucleation. (Reproduced from Moon, C., Taylor, P.C., Rodger, P.M., J. Am. Chem. Soc., 125, 4706 (2003). With permission from the American Chemical Society.)... 

In order to verify which of the above nucleation mechanisms accurately represents hydrate nucleation, it is clear that experimental validation is required. This can then lead to such qualitative models being quantified. However, to date, there is very limited experimental verification of the above hypotheses (labile cluster or local structuring model, or some combination of both models), due to both their stochastic and microscopic nature, and the timescale resolution of most experimental techniques. Without experimental validation, these hypotheses should be considered as only conceptual aids. While the resolution of a nucleation theory is uncertain, the next step of hydrate growth has proved more tenable for experimental evidence, as discussed in Section 3.2. 

Hydrate nucleation (the initiation of growth, occuring during the induction period) is a stochastic process (with significant scatter in the data at low driving force under isothermal conditions). 

The time-dependent phenomena of hydrate nucleation and growth are challenging to both measure and model. This is in contrast to hydrate thermodynamics that... 

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. 

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. 

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