Hydrate Phase Equilibria of Phosphonate Scale Inhibitors, Amines, and Ethylene Glycol

Abstract

Hydrate formation is a serious concern in the production and transportation of natural gas. While there exist numerous hydrate inhibitors, there are a plethora of other chemicals that are also present alongside monoethylene glycol (MEG) for various reasons such as scale, wax, corrosion, and emulsion inhibition along the pipeline. These chemicals have not been modeled in hydrate simulators nor have their impact upon gas hydrate been studied experimentally. Thus, in this study, three scale inhibitors, iminodi(methylene) phosphonate, nitrilotris(methylene) phosphonate, and diethylenetriaminepenta(methylene) phosphonate, and two amines, monoethanolamine (MEA) and diethanolamine (DEA), were tested for methane hydrate formation using the isochoric method. The average temperature depression for each chemical as mentioned in the aforementioned order was found to be 0.06, 0.15, and 0.2 K for the scale inhibitors at a concentration of 350 ppm. This suggests that scale inhibitors may also inhibit hydrate formation, albeit at limited extent, but more importantly, they do not serve to promote hydrate formation; thus, they are not disturbing the hydrate control program. While for the amines, an average temperature depression of 0.2 and 0.47 K was found for MEA and DEA at a concentration of 5 wt %, respectively, suggesting that such amines when used alongside MEG may bring about an additional hydrate inhibitory performance.