www.acsprf.org

In subsea pipelines, mixtures of natural gas, co-produced water, and liquid hydrocarbons (condensate) can be subjected to conditions where hydrate formation is thermodynamically favorable. The formation of hydrates in these circumstances is highly undesirable; the hydrates plug pipelines and are costly and hazardous to remove. Often, thermodynamic inhibitors are added to lower the hydrate formation temperature; however, this requires large amounts of solvent such as methanol. An alternative is to use low dosage hydrate inhibitors that can prevent plug formation at low concentrations and for which there are many economic and environmental advantages. Specifically, there has been much interest in anti-agglomerants – surfactants that act by preventing adhesion between hydrate particles. However there is not a good, fundamental understanding of how anti-agglomerants affect hydrate adhesion and plug formation.

This project involves the study of adhesion between hydrate particles and model particles. The goal is to better understand the effects of surfactants (anti-agglomerants) on the interactions between particles, by examining the flow behavior of particle slurries. The work will provide a better fundamental understanding of (1) the role of capillary forces on adhesion between particles and the flow behavior of the slurries and (2) the effects of surfactants on the adhesion between particles.

The work over this time period focused on continuing the study of the effects of capillary forces on the flow behavior of model suspensions and on the effects of surfactants on capillary forces. The model system consisted of glass particles dispersed in mineral oil. It was thought that capillary forces from liquid water bridges between glass particles and suspensions containing these particles would be similar to the capillary forces between hydrate particles and the dispersions of hydrates that adhere due to capillary forces. Thus understanding how capillary forces influence the flow behavior of a suspension and how surfactants affect this behavior would give some insight into the hydrate slurries and how certain surfactants may prevent agglomeration. One undergraduate student worked on this project.

Dispersions with smaller glass particles (~5 mm) were studied and found to produce very different behavior from the larger particles (~40 mm). The viscosity profile for the smaller glass particles showed that there is significant particle interaction even in the absence of liquid bridges – the flow profile for the larger particles was nearly Newtonian while it was shear thinning with much higher viscosities for the smaller particles. The effect of water on the flow behavior of suspensions of glass microspheres dispersed in mineral oil was investigated for various levels of water and particle volume fractions. Just as it did for the larger particles, the addition of small amounts of water led to large increases in viscosity due to the formation of water bridges between particles. The flow profiles were highly shear-thinning.

Hydrophobic surfactants, Span 80 and Span 20, were found to reduce the viscosity of the dispersions (both with and without water), and the dispersions would be only slightly shear-thinning at high enough surfactant concentrations.

Dispersion viscosity models are currently being studied to see how well the flow profiles can be described using these models.

This work represents one of the only studies on the effects of capillary forces on the flow behavior of suspensions. The results may give some insight as to why some surfactants are more effective anti-agglomerants of gas hydrates. It would be interesting to see if model systems (such as glass spheres rather than hydrate particles) could be used to screen potential anti-agglomerants.

Two undergraduate students presented results from the previous (2010) report year at the 2010 AIChE Annual Meeting, and these two students were also co-authors with the PI on a paper published in AIChE Journal. One undergraduate student is expected to present results from the 2011 report year at a conference later this year.