AmericanChemicalSociety.com

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. 

Most of the work over this time period focused on studying 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. Two undergraduate students worked on this part of the project.

The effect of water on the flow behavior of suspensions of glass microspheres dispersed in mineral oil was investigated for various levels of water, particle volume fractions, and particle sizes. The addition of small amounts of water led to large increases in viscosity due to the formation of water bridges between particles. The capillary forces between the particles also made the flow profiles highly shear-thinning across the range of particle volume fractions that were studied (0.10 to 0.25). The presence of water led to a significant effect of particle size, where dispersions of smaller particles were much more viscous than those of larger particles. This was not the case in the absence of water. Also, the amount of water that was added affected the viscosity, where the viscosity of a dispersion goes through a maximum as the amount of water is increased.

Two hydrophobic surfactants, Span 80 and Arquad 2HT, were found to reduce the viscosity of the dispersions with water added and cause the flow profile to be less shear-thinning. The increase in Span 80 concentration from 0.05 to 1.0 wt% gives rise to a fairly gradual decrease in viscosity by both lowering the interfacial tension and reducing the number of liquid bridges. The addition of Arquad 2HT at even 0.05 wt% reduces the viscosity of a dispersion to the same level as that for a dispersion with no water added by preventing liquid bridges from forming through an increase in the contact angle of water on glass in the presence of oil. A number of other surfactants were also studied, most with only a small effect on suspension viscosity.

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 surfactants like Arquad 2HT (quaternary ammonium salts) are more effective anti-agglomerants of gas hydrates as compared with certain nonionic surfactants like Span 80. 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.

In addition to the glass microsphere study, one undergraduate student began to develop the experimental procedure to examine the effects of various surfactants on the agglomeration of THF hydrates in an oil/water system under flow. Agglomeration was monitored by measuring the torque on a mixer or the torque in a viscometer over time. Preliminary studies showed that Span 80 was able to delay the onset of hydrate agglomeration, but it would still occur. Arquad 2HT in some cases was able to prevent large-scale agglomeration. The experimental method for these studies is still under development.

Two of the undergraduate students are expected to present results from their work at a conference in the Fall of 2010. These two students are also co-authors with the PI on a paper submitted for publication.