Page 120 - Handbook Of Multiphase Flow Assurance

116 5. Flow restrictions and blockages in operations

FIG. 5.15 Increased corrosion in stainless steel observed during hydrate formation (Makogon, 1997).

Methanol in small amounts (below 2 wt%) acts as a hydrate growth rate promoter and may
act as enhancer for methane and natural gas hydrate. The subcooling required to start hydrate
formation is reduced when a small amount of methanol is added to water, at pressures be-
tween 50 and 100 bar (Makogon, 1974, 1981).
In case of insufficient LDHI, hydrate plug forms more rapidly with too little kinetic inhib-
itor than without a kinetic inhibitor.
With too little antiagglomerant inhibitor formed hydrates agglomerate into a plug.
• Inhibitor evaporation to gas
Volatile inhibitors (methanol) can evaporate into the gas phase, leaving the water
underinhibited.
The method to estimate the rate of methanol loss from water to gas is provided by Sloan
(1990, 2000) as 1 pound of methanol per MMscf gas for every weight percent of methanol in
water phase, at pressures >1000 psi. For more accurate estimates, the following three correla-
tions were offered by Sloan (2000):
(mol frac. methanol in gas/mol frac. methanol in water) = exp(8.412–7250/T[°R]) for
1000 psia.
(mol frac. methanol in gas/mol frac. methanol in water) = exp(6.852–6432/T[°R]) for 2000
psia.
(mol frac. methanol in gas/ mol frac. methanol in water) = exp(5.706–5738/T[°R]) for
3000 psia.
For glycol, 0.02 pounds of MEG is lost to gas per MMscf gas, at pressures >1000 psi.