Page 236 - Handbook Of Multiphase Flow Assurance

Hydrate stability and crystal growth 235

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Rates of linear hydrate growth may reach up to 10 nm/s as reported by Makogon (1974,
p. 74). This allows one to estimate the time required to orient one layer of water molecules into
hydrate lattice. Taking the size of water molecule as its van der Waals diameter of 0.29 nm, it
takes 290 μs for water molecules to position themselves from bulk water into the crystalline struc-
ture. This scale of time is about 10 times larger than time scale of usual computer simulations in
regular computers using distributed processing or GPUs with CUDA or similar parallel coding
and achievable in supercomputers. This indicates that, most likely, kinetics of hydrate formation
will be studied experimentally and using computer molecular modeling in the near future.
In experimental work kinetics of gas hydrate formation may be affected by many different
factors. Among these are:
(a) Subcooling, or lowering the system temperature below the equilibrium value for a
given pressure. Different shapes of hydrate crystals were obtained by Makogon (1981,
pp. 88–100) depending on the amount of subcooling.
(b) Overpressurizaton, or increase of the system pressure above its equilibrium value for a
set temperature. This is another measure of subcooling.
(c) Rate of cooling, or gradient of temperature decrease of the system in time.
(d) Stirring rate. Effect of turbulence in the hydrate system on kinetics of crystallization,
the crystal size distribution and the duration of the induction period was described by
Englezos et al. (1987).
(e) Previous temperature of water available for hydrate formation. This effect was studied
by Makogon (1981, pp. 63–72).
(f) Presence of the sites for hydrate nucleation like steel walls of the reactor or pipeline, or
particles of silica or bentonite.
(g) Preliminary saturation of water with hydrate forming gas. Dissolution of gas in water is
a diffusion process if no mixing were applied to the system. Rate of gas dissolution may
be monitored and subtracted from total gas consumption during the experiment.
Kinetics of hydrate formation is strongly affected by so- called kinetic inhibitors. The
mechanism of kinetic inhibition is by adsorption to hydrate nuclei and by sterically blocking
guest molecules from reaching the hydrate surface. Kinetic inhibitors are usually polymeric
molecules of high molecular weight having the ability to hydrogen bond with water.
• Comparison of chemical performance on a crystal solid surface and laboratory methods

Phase transitions in gas hydrates
For a single guest component hydrate, a solid phase transition can be indicated by a sharp
change of slope in the three-phase, univariant pressure-temperature equilibrium line shown
as a discontinuity in a plot of d(ln P) against d(1/T). This slope change is due to the change
in the enthalpy of hydrate formation, Δ d H, determined by the Clausius-Clapeyron equation
applied to a univariant system.

(
/ ZR)
d ln P) / d(1 /T) = ∆ d H ( (10.4)
The univariant Clausius-Clapeyron equation is given in the above equation where P and T
are absolute pressure and temperature of hydrate equilibrium with vapor and ice, Δ d H is the
enthalpy of dissociation of hydrates to ice and vapor, Z is the compressibility of the gas, and