Hydrate formation and particle distributions in gas–water systems

• Published in: Chemical engineering science 2013-12, Vol.104, p.177-188
• Main Authors: Akhfash, Masoumeh, Boxall, John A., Aman, Zachary M., Johns, Michael L., May, Eric F.
• Format: Article
• Language: English
• Subjects: chemical engineering; Gas hydrate; methane; Particle distribution; prediction; turbulent flow; Under-inhibition; Water-dominated
• ISSN: 0009-2509; 1873-4405
• DOI: 10.1016/j.ces.2013.08.053

Methane hydrate formation rate and resistance to flow were measured for gas–water systems in a high-pressure visual autoclave over a range of mixture velocities (300–5000 Reynolds number). A transition from a homogeneous to heterogeneous particle distribution, proposed following flowloop studies by others, was observed directly in the autoclave through three independent measurements: motor current increase (resistance to flow), pressure consumption rate (hydrate growth rate), and visual observation. The hydrate volume fraction at the transition, ϕtransition, generally increased with increasing turbulence, although the relationship between Reynolds number and ϕtransition was not the same as that observed in flowloop experiments. The addition of a thermodynamic inhibitor below the full inhibition threshold (i.e. under-inhibited) increased the transition point by about 10vol% hydrate, without affecting the initial hydrate growth rate. A simple mass transport-limited formation model with no adjustable parameters was implemented to enable quantitative predictions of hydrate formation rate. In sufficiently turbulent systems the model's predictions were in excellent agreement with the observed growth rates. At lower Reynolds numbers, two mechanisms are proposed to explain the deviations between the observed and predicted growth rates. Prior to ϕtransition the low shear means that hydrate formation is limited by the rate at which the aqueous phase can be re-saturated with methane. This rate is increased greatly by the formation of a hydrate bed after ϕtransition, which increases the gas–liquid interfacial area. Relative motor current (resistance to flow) increase in high-pressure autoclave as a function of methane hydrate volume fraction, for three repeat trials at an initial Reynolds number of approximately 2460. The regional boundaries (A–D) arise from slope changes in the average hydrate formation rate; resistance to flow was observed to increase only after approximately 14vol% hydrate, and large fluctuations in motor current were observed after approximately 47vol% hydrate. [Display omitted] •Methane hydrate was formed in a high-pressure visual autoclave.•A transition from homogeneous to heterogeneous particle distribution was observed.•The transition points agreed from visual, pressure, and motor current measurements.•Under-inhibition increased the transition point by approximately 10vol% hydrate.