Hydrate Inhibitors Question

[Pandamort]

Good afternoon

I was wondering why methanol is used instead of glycol as a hydrate inhibitor, although glycol has more advantages?

I need a short answer . Any help will be appreciated.

Many thanks

[gilbert]

Methanol or Mono Ethylene Glycol so its a Glycol and injected to flowlines, TEG is used in dry gas.

[Pandamort]

So far, I know this:

Selection criteria to be considered:

• Gas Dehydration Capability – Methanol will not dehydrate gas to pipeline specification
(typically 7 lb/MMSCF). MEG and the other glycols have this capability that eliminated the
requirement for a TEG contactor on the topsides.

• Chemical Cost – Methanol is less expensive per gallon than MEG although its price varies
significantly. The price of MEG is fairly stable. The controlling factor is the product of
chemical cost times gallons consumed, thus chemical losses rather than price per gallon can
have the greatest bearing on chemical costs.

• Physical Properties – The viscosity of methanol is at least an order of magnitude less than
that of MEG, thus reducing the pressure drop and pumping horsepower required for
injection. The lower molecular weight of methanol (32 versus 62 for MEG) allows more
dilute methanol solutions to be used since hydrate temperature depression is a molar
phenomenon. However the amount of methanol in the gas phase must be also considered
when calculating methanol injection rates.

• Safety – Methanol poses greater safety risks in handling and storage than MEG. For
example, flash points of methanol and MEG are 52 and 232oF, respectively.

• Corrosion Inhibition of Flowlines – Typically MEG provides a greater degree of corrosion
protection than methanol.
Solubility Losses – Solubility loss of MEG into the sales gas is negligible and loss to the
liquid hydrocarbon phase is very low. Depending on operating conditions, the solubility loss
of methanol into sales gas can be very high, typically 1 lb of methanol/MMSCF for every %
methanol in water phase. Losses to the liquid hydrocarbon are higher than for MEG but
usually less than 1-2% of hydrocarbon volume. Depending on solubility losses chemical
makeup requirements for methanol can be very large and expensive for both once-through
systems and methanol recovery units.

• Tolerance to Salts – Both methanol and glycol solutions can become contaminated with salts.
In recovering methanol by distillation, any salts present distribute to the aqueous phase since
methanol is more volatile than water. The reverse is true when regenerating glycol. Since
water is more volatile than glycol the salts remain behind in the glycol phase during
regeneration. To avoid salt precipitation and plugging problems, it will be necessary to
remove salts from the glycol solution.

• Paraffin Inhibition – MEG has paraffin inhibition/dispersion properties while methanol has at
best exhibits very little inhibition properties. MEG if present in sufficient quantities relative
to the volume of condensate or oil inhibits paraffin deposition by forming an aqueous
continuous emulsion phase.

• Environmental Issues – Both MEG and methanol can contribute to toxicity of the overboard
produced water discharge. In some overseas locations (e.g. North Sea) the methanol
concentration of water discharged is limited to 1000 ppm maximum.

• Solid Waste – No solid waste is generated in methanol recovery processes. Glycol reclaiming
processes can generate a solid or concentrated liquid waste product, primarily salt, for
disposal.

• Downstream Issues – As previously stated a significant amount of methanol would be lost to
the hydrocarbon phases, which may cause problems for the refineries and gas plants
downstream. In gas plants where there is propane recovery the methanol will follow the
propane product and potentially cause the propane to go off spec. Methanol has also been
known to cause premature failure in mol sieves. In refineries the methanol must be washed
out of the crude/condensate, where it presents a problem in wastewater treatment. For glycol
systems there is virtually no loss to gas phase, and the solubility of glycol in the hydrocarbon
liquids is typically 1 to 2 orders of magnitude less than methanol.


But, I still don't know why methanol is more common than MEG or other glycols. They seem better for me. Please clarify this to me. Thank you

[cybermann]

Methanol is not recoverable, and we use in plant when hydrate form in valve or other equipemnt, we use methanol when deshydratation at gas processing use molecular sieve

[ameer_mechanical]

the methanole incresse the heating valve of the gas and that is another advantage