Thermodynamic hydrate inhibitors

Thermodynamic hydrate inhibitors (methanol, glycols) are widely used as a primary hydrate inhibitor in subsea systems. For projects producing oil, the amount of methanol is limited in terms of the oil’s quality. The issue of an oxygenated solvent limit for glycols in oil is still under discussion. Even though the recovery units of methanol and glycol are improving, the units
File:MeOH Recovery Flowchart of Offshore Gas Facility.png
MeOH Recovery Flowchart of Offshore Gas Facility
require appreciable heat to recover the THIs, and scaling in the methanol and glycol stills can generate operations challenges, especially when the produced water chemistry is not available from the reservoir appraisal during the design phase.

Glycol recovery units can be designed to remove the salts that have traditionally limited the glycol quality. To reduce the methanol and glycol to the requirements of oil quality, crude washing requires large volumes of water that must be treated to seawater injection quality. The recovery units and wash units have a significant weight and can affect the project’s design. These units are large and heavy.

Subsea flowlines enter the separation vessels and are then distributed in three separate phases:

  • Gas export stream to onshore gas plant in vapor phase;
  • Condensate or oil export stream to shore in liquid hydrocarbon phase;
  • Produced water stream in aqueous phase.

The methanol in the hydrocarbon vapor phase is recovered by adsorption, and the methanol in the hydrocarbon liquid phase is recovered by water wash, mechanical separation, or a combination of the two. Most of the injected methanol is in the aqueous phase. A methanol tower is used to recover the methanol from the aqueous phase. More than 96% of the methanol injected can be recovered if good engineering judgment and experience are applied.

The aqueous phase mixture contains most of the methanol injected to the system. MetFhanol is a highly polar liquid and is fully miscible with water; therefore, the recovery of methanol from water is achieved by distillation rather than phase separation.

Methanol is fully miscible with water, while the solubility of methanol in hydrocarbons is very small. Therefore, water (the solvent) can be used to extract methanol (the solute) from the hydrocarbon condensate (the feed) efficiently. Because water is the solvent, this extraction process is called water wash.

Methanol losses in the hydrocarbon liquid phase are difficult to predict. Solubility of methanol is a strong function of both the water phase and hydrocarbon phase compositions. The recovery of the dispersed and dissolved methanol from the feed to the condensate stabilizer can be achieved by mechanical separation via coalescence, liquid–liquid extraction, or a combination of the two depending on the feed characteristics,weight, space, and cost of the entire subsea topside operation.

Mechanical separation can separate the dispersed methanol from the hydrocarbon liquid phase to a certain extent. Liquid–liquid extraction can recover the smaller droplets of dispersed methanol and the dissolved methanol from the hydrocarbon liquid phase.However, liquid–liquid extraction is more costly than mechanical separation. The solvent needs to be regenerated and reused by the extraction tower.

It increases the flow rate and the heating and cooling duties of the methanol distillation tower. For subsea operation, the treating facility’s space requirements, weight, and cost have to be considered together to determine which system to use to recover the methanol from the liquid hydrocarbon phase. Methanol in the hydrocarbon vapor phase can be recovered by adsorption. If the hydrocarbon vapor passes through a cryogenic system, the methanol in the hydrocarbon vapor phase condenses into the liquid phase and can be recovered.


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