Page 288 - Handbook Of Multiphase Flow Assurance

Experimental and computer study of the effect of kinetic inhibitors on clathrate hydrates 287

oxide (11.1 °C (Glew and Rath, 1966) which forms sI. H2S forms sII hydrate at 0.5°C (Ward
et al., 2015) at atmospheric pressure.
A study of natural gas hydrate crystals growth for sI, sII and sH was recently performed
by Smelik and King (1997). In their work crystals of methane hydrate (sI), methane + propane
hydrate (sII), and methane + methylcyclopentane hydrate (sH) were grown in a visual reactor
under pressure. Structure I crystals generally exhibited {110} and in some cases {100} faces.
Structure II crystals grew as octahedra exhibiting {111} faces. Structure H crystals grew as
hexagonal prisms with {001}, {110}, and {120} faces.
Similar work on the growth of the cubic type crystals of pyrite was performed by
Murowchick and Barnes (1987). They studied the effects of temperature and degree of su-
persaturation on morphology of pyrite crystals. Two types of crystal growth were observed:
surface- controlled growth when the rate of growth is limited by the rate at which compo-
nents are incorporated into the growing surface, and the diffusion-controlled growth when
the growth rate is limited by the rate at which nutrient components diffuse to the surface.
Growing faces of the crystals were smooth in the surface-controlled regime, when the nu-
trients were equally available to faces, edges and vertices of the crystal. In the diffusion-
controlled case protrusions such as vertices and edges were in contact with a nutrient- richer
solution. Growth at the end of protrusions was favored over growth on the flat surface. Such
a mechanism resulted in striated and dendritic growth.

Inhibition of hydrate formation

Hydrates of natural gas can plug flow channels (wells and pipelines) during production
of natural gas. There are four classical methods for preventing hydrate formation in a system
containing a hydrocarbon gas: increasing temperature of the system, decreasing system pres-
sure, removing water from the system, or adding inhibitors of hydrate formation.
“Thermodynamic” hydrate inhibitors such as alcohols, glycols or salt are commonly used
to avoid hydrate formation in gas and oil industry for systems where dehydration or heating
are impossible or not economic. Such inhibitors shift the thermodynamic stability boundary
of hydrates to higher pressure or lower temperature by aggregating with water molecules
and preventing their arrangement into a hydrate lattice (Sloan, 1990). This method is not al-
ways the best environmental or economic solution for preventing hydrate plugs.
Recently, the CSM Center for Hydrate Research proposed an alternative method of inhib-
iting gas hydrates. It was found that certain polymers, when added to water, will delay the
conversion of gas and water into hydrate. The induction time for the onset of hydrate forma-
tion is generally unpredictable, or stochastic at fixed conditions. The same stochastic behavior
is observed for the onset of freezing in a pure water system seeded with AgI crystals (Barlow
and Haymet, 1995). However, the hydrate induction time is a function of supersaturation or
pressure (Herri et al., 1996).
Numerous researchers have reported that additives affect the crystal morphology. Michaels
and Colville (1960) reported that surfactants inhibit certain faces of adipic acid crystals grow-
ing from aqueous solution. Cationic surfactants caused greater reduction in the growth rate of
{010} and {110} faces. Anionic surfactants had such effect on the {001} face. These effects were
related to the hydroxyl density on the crystal faces.