(114a) Reducing the Adhesion of Hydrates Using Durable Hydrate-Phobic CVD Polymers

The formation and accumulation of clathrate hydrate inside oil and gas pipelines causes severe problems in deep-sea oil and gas operations. Hydrate-phobic, durable, and mechanically-robust bilayer poly-divinyl benzene (pDVB)/poly(perfluorodecylacrylate) (pPFDA) coatings were developed using initiated chemical vapor deposition (iCVD) technique to inhibit the formation of hydrates and in the case of any formation reduce their adhesion to underlying substrates. Optical profilometer measurement was performed on the coating and root mean square (Rq) value of 18.9±5.4 nm was obtained. Mixtures of tetrahydrofuran (THF) and water were used to study the formation of hydrates and their adhesion strength. The weight percentage of the THF in water was changed from 0-70 and resulted in a decrease in surface tension of THF-water mixture from 70 to 28 mN/m. Goniometric measurements on coated substrates exhibited advancing water contact angle of 157.8º ± 2.3º and receding water contact angle of 131º ± 8º, whereas 70 weight % THF in water presented advancing angle of 85.1º ± 6.1º and receding angle of 48.5º ± 4.9º.

A 20 weight % THF in water mixture was found to be critical to form only hydrate when cooled down to low temperature at atmospheric pressure. The strength of hydrate adhesion experienced ten-fold reduction when substrates were coated with these iCVD polymers: from 1050±250 kPa on bare substrates to 128±100 kPa on coated ones. A weight % of THF in water lower than 20 resulted in formation of both ice and hydrate when cooled to low temperature. In this regime, a relation was obtained to express the adhesion strength of ice/hydrate mixture as a function of ice adhesion strength and hydrate adhesion strength. The adhesion strength of ice, hydrate, and ice/hydrate mixture were found to correlate well with the work of adhesion between liquid droplets used in their formation (water or THF-water mixture) and underlying substrate.

The weight % of THF in water increased above 20 and resulted in partial formation of hydrate with some unfrozen THF-water mixture. In this regime, the adhesion strength of hydrate was found to correlate well with the work of adhesion between 20 weight % THF in water droplets and underlying substrate. These data suggest that 20 weight % THF in water has to be used as a probe fluid to identify the strength of hydrate/unfrozen liquid adhesion to the substrates.

The impact of subcooling temperature and time on the adhesion strength of hydrate on coated and bare substrates was studied. These data suggest that the combination of time and subcooling temperature is important to form THF hydrates. However, once the hydrates formed, their adhesion strength were comparable regardless of their formation conditions. The results of this work suggest that THF-water mixture repelling nature of a given substrate can be utilized to assess its hydrate-phobic behavior; hence, it opens a new pathway for studying hydrate-phobicity.