Gas hydrate anti-agglomerant properties of polypropoxylates and some other demulsifiers

Abstract

Low dosage hydrate inhibitors (LDHIs) have been developed over the last 15 yr as a new gas hydrate control technology for the oil industry, which can be more cost-effective than traditional practices such as the use of thermodynamic inhibitors e.g. methanol and glycols. Two classes of LDHI called kinetic inhibitors (KHIs) and anti-agglomerants (AAs) are already being successfully used in the field. This paper discusses new types of AAs, which are based on surfactants with a high degree of propoxylation. High pressure tests in sapphire cells show that polyamine polypropoxylates and other branched polypropoxylates are able to disperse gas hydrates in a hydrocarbon fluid as long as there is good agitation in the fluids. Formation of an emulsion is not required for this AA effect. Linear, unbranched or low molecular weight polypropoxylates did not perform well under the same conditions, as well as many other surfactant classes including anionic and various polyethoxylated surfactants. Some polyamine polypropoxylates gave weak kinetic hydrate inhibition effects. Addition of kinetic hydrate inhibitors such as polyvinylcaprolactam reduces the performance of polyamine polypropoxylates as AAs. Part of the mechanism for the AA effect of the surfactant polypropoxylates is proposed to rely on their low solubility in both the aqueous and hydrocarbon phases. The surfactant polypropoxylates form a separate layer between the two phases, which coat the dispersed water droplets as they are converted to gas hydrates, keeping them from agglomerating. After shutting in the cell by stopping the stirring for some hours the hydrates can be redispersed again have a high interfacial concentration, were tested as AAs. Some other demulsifiers showed the same anti-agglomeration properties as the polypropoxylates. One demulsifier, Dowfax DM655, an alkylphenol formaldehyde resin alkoxylates, gave good AA performance at up to 16.5 °C subcooling when dosed at 10,000 ppm in synthetic sea water. However, the performance decreased at low salinity (≤ 0.5 wt.%) and at water cuts of 35% or more.