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Wetting and film growth phenomena for cyclopentane hydrates and potential non-plugging behavior
Stoner, Hannah Marie
Stoner, Hannah Marie
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2023
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2024-11-29
Abstract
Clathrate hydrates are crystalline inclusion compounds composed of hydrogen bonded water cages in an ice-like structure that trap guest molecules, typically light hydrocarbons (e.g., methane) at high pressures and low temperatures. A major application of gas hydrates is in the field of flow assurance, where they can cause blockages in petroleum flowlines. To advance understanding of hydrate mitigation strategies, study of the hydrate interface and how this may affect hydrate agglomeration is important. This thesis presents new findings relating the wetting and film growth behavior at the hydrate interface in the presence of different physicochemical conditions for cyclopentane hydrates, a common low-pressure analog to natural gas hydrates.
The effects of the state (i.e. roughness) of the hydrate interface and annealing time of the hydrate on the surface wettability (quantified through direct measurement of the contact angle) coupled with film growth behavior were investigated. The results were also used to reconcile differing literature values reported for the value of the contact angle of a DI water droplet on a cyclopentane hydrate surface. The discrepancies were attributed to contrasting surface roughness due to differing techniques used to prepare the hydrate samples. The results showed that the wettability decreased (i.e. contact angle increased) with increasing annealing time thought to be because of a ‘drier’ hydrate surface that allowed for less spreading. Conversely, the film growth rate remained the same regardless of annealing time due to no change in kinetic driving force across experiments (i.e. subcooling temperature). It was also shown that the wettability generally decreased as subcooling increased due to increased surface roughness making it more difficult for the water droplet to spread. This effect was balanced by the film growth which increased with an increasing subcooling temperature because of a larger kinetic driving force. The concept of the crystallization angle was also introduced, which refers to the angle created by the now solid hydrate shell covering the droplet at the original three phase point.
To further understand the behavior of the hydrate interface, and particularly how it is affected by oils (containing natural surface active molecules), the contact angle and film growth rates were determined for one model oil and 17 natural oils. These tests were performed on cyclopentane hydrates containing 0.02 vol.% of each oil. It was shown that there is a potential relationship between lower hydrate surface wettability and a higher asphaltene content in the oil. A relationship was also found for only specific oils between lower wettability and slower film growth rate, indicating a possible kinetic inhibition effect from the natural surfactants in the oils. Both behaviors were attributed to alignment and adsorption of the natural AA molecules (thought to be asphaltenes) at the hydrate/hydrocarbon interface. A concentration scan was also performed for a select six oils, which showed increasing the oil concentration in cyclopentane resulted in a lower wettability (i.e. higher contact angle) and slower film growth rate which further supports the hypothesis of surfactant alignment and adsorption at the interface. Four oils were fractionated and studied further to observe the effect from the asphaltene + binding resin fraction, the asphaltene fraction, and the deasphalted fraction compared to the whole oil. This showed that the fractions with more surface active (i.e., polar) components had larger contributions to decreasing wettability and film growth rate, depending on the aggregation state of the asphaltenes.
The study of interfacial behavior was also expanded to include the effect of different salts. The effect that some chloride salts had on wettability, film growth, and cohesive force were tested for cyclopentane hydrates. Overall, for salts with a lower concentration (< 1.5 wt%) in the water droplet for contact angle/film growth and in the hydrate for cohesive force, trends were unclear due to competing effects of low salt concentration causing possible hydrate promotion and inhibition effects of the salt on the hydrate phase boundary. For higher salt concentrations, the addition of more salt resulted in an increased wettability (i.e., reduced contact angle) and reduced film growth rates. This behavior was attributed to local shifts in the equilibrium temperature and resulting surface melting. Cohesive forces were also reduced due to visual surface roughness disrupting the capillary bridge. Some select oil and salt combinations were also tested. Wettability and film growth results showed how salt can act antagonistically or synergistically depending on its interaction with the oil (natural and model).
The role of particle size analysis in hydrate agglomeration prediction is also presented, with a sensitivity analysis of typical cohesion force and agglomeration models, i.e., the Capillary Bridge (CB) equation and the Camargo and Palermo (C&P) model. Two existing flowloop data sets were analyzed as case studies to understand how water droplet or particle size evolves over the course of a flowloop experiment. The hydrate particle data set was manually analyzed to obtain the parameter of particle diameter (dP) for the CB and C&P models to calculate the agglomerate diameter (dA) and cohesion force (FA). Such analyses can help with further refinement of the hydrate kinetic models, where dA and FA are major inputs.
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