Hydrate nucleation and growth

A trick to accelerate the simulation is to impose order parameters relevant to the description of the crystalline state to drive the system towards nucleation. This method was applied to the study of water nucleation and carbon dioxide hydrate formation using a conventional Monte Carlo method. ° ° Stein-hardt s bond-orientational order parameters Q and W , based on quadratic and third-order invariants formed from bond spherical harmonics (Y/ ,(0, (p)), were employed. These order parameters allow quantitative measures of the local symmetry in liquids and glasses. The rotationally invariant orientational order parameters are defined as  

N is the number of nearest-neighbour contacts within a given radius cutoff. Wi is defined as 

For water molecules an additional order parameter needed to measure 

It is preferable to observe nucleation under equilibrium conditions free of constraints. With the advent of computing technology in recent years, direct observation of crystallization in an equilibrium MD simulation became feasible. The nucleation and growth of an ice crystal have been reported from a canonical ensemble constant volume-constant temperature (NVT) MD simulation in 2002. The calculations were performed on 512 water molecules using the TIP4P ° water potential. The density of the water was 

The mechanisms of nucleation followed by subsequent growth of the ice crystal discussed below are similar to that observed in the simulation of homogeneous nucleation of methane hydrate. More importantly, results of MD simulations show hydrate formation always initiate near the water/methane interface where there is a large concentration gradient difference exists between the methane gas and the solution. This was observed in slab calculations that have a distinct water/methane interface and around the methane bubble in the spontaneous nucleation study. The nucleation prediction is 

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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. 

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. 

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. 

To borrow Churchill s phrase, proving such hydrate phase kinetics measurement principles represents the end of the beginning for time-dependent experiments. The systematic kinetic study to generate a fundamental mechanism of hydrate nucleation and growth constitutes the major remaining challenge in hydrate physico-chemical science. 

There have been many instances of examination of the effect of additive product on the initiation of nucleation and growth processes. In early work on the dehydration of crystalline hydrates, reaction was initiated on all surfaces by rubbing with the anhydrous material [400]. An interesting application of the opposite effect was used by Franklin and Flanagan [62] to inhibit reaction at selected crystal faces of uranyl nitrate hexa-hydrate by coating with an impermeable material. In other reactions, the product does not so readily interact with reactant surfaces, e.g. nickel metal (having oxidized boundaries) does not detectably catalyze the decomposition of nickel formate [222],... 

Measurements of the kinetics of the individual nucleation and growth steps in the reactions of several hydrated sulphates have been referred to in Sect. 1.2 though, perhaps surprisingly, these data were not combined in a kinetic analysis for the overall reaction in studies of the alums [51,431, 586] or NiS04 7 H20 [50]. Indeed, Lyakhov and Boldyrev [81], in one of the few reviews of the field, maintain that the satisfactory topochemi-cal description of dehydrations is a problem which at present remains... 

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]. 

Englezos, P. (1996). Nucleation and growth of gas hydrate crystals in relation to "kinetic... 

Moudrakovski, I.L. Sanchez, A.A. Ratcliffe, C.I. Ripmeester, J.A. (2001). Nucleation and Growth of Hydrates on Ice Surfaces New Insights from 129Xe NMR Experiments with Hyperpolarized Xenon. J. Phys. Chem. B, 105, 12338-12347. 

These processes may be affected by chemical admixtures, particularly the formation and properties of the protective layer. Also, admixtures remaining in the pore solution may further influence nucleation and growth of the hydration products, causing volume expansion, outward mechanical pressure on the protective gel layer and its subsequent disruption. 

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. 

The evidence from microstructure, calorimetry and other sources suggests that the hydration processes of cement and C3S are essentially similar. There are important differences in the nature of the early product and in where the C-S-H formed in the middle stage of reaction begins to deposit, but in both cases it would appear that the early reaction slows down because of the deposition of a layer of product, which either isolates parts of the anhydrous surfaces from the main solution or allows the concentrations close to those surfaces to rise to values approaching the theoretical solubilities of the anhydrous compounds. In both cases, the initiation of the main reaction and the kinetics in its acceleratory phase appear to be controlled by the nucleation and growth of C-S-H. 

Han et al. (2008) reported the s)mthesis of heavy lanthanide sesqui-oxide (R2O3, R = Y, Dy, Ho, Er) nanobelts by thermolysis of solid rare earth nitrate hydrates in a dodecylamine/l-octadecene mixed solvent system. The nitrate hydrates showed poor solubility in the mixed solvent, and the heat-transport differences between the liquid and the solid assisted in separation of the nucleation and growth processes. 

Early (1930 to 1940) kinetic studies of dehydrations contributed much to the development of the concept of the reaction interface as the important feature of nucleation and growth reactions [2]. Kinetic equations applicable to the decompositions of a vnde range of crystalline substances were developed. Large, well-formed crystals of hydrates could be prepared relatively easily and studies of these were particularly rewarding. The interpretation of kinetic data was supplemented by microscopic evidence concerning the formation and development of product nuclei. Recent work on dehydrations has included more precise determinations of the crystal structures of reactants, products and their interrelationships, including interface textures, in the attempt to resolve unanswered questions. 

Dunning [32] has reviewed earlier interpretations of nucleation and growth phenomena in crystalline hydrates and has developed a theoiy of product phase... 

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