Naturally-occurring methane hydrates have attracted a lot of public attention because they provide enormous potential as a future energy source. Gas storage in the artificial clathrate form is a safe and economical option, safe because the slow release of gas from a container of hydrate reduces explosion hazards, and economical because the gas storage density at 40 bar is comparable to a compressed gas at 200 bar. On the other hand, hydrate plugs formed inside processing lines are a nuisance to gas and oil industries. A critical obstacle to accelerating or retarding hydrate formation is a lack of understanding of the multi-scale interactions between gas hydrate particles and surface-active agents during hydrate formation. To contrast these interactions, methane (CH4) and carbon dioxide (CO2) hydrate systems will be investigated because surface-active agents generally accelerate CH4 hydrate formation but do not affect or even hinder CO2 hydrate formation. Three sequential stages of hydrate formation will be analyzed at different length scales: nucleation (<100 nm), initial growth (> 1 ìm), and layered & agglomerated growth (> 1 mm). Differential scanning calorimetry will be used to quantify the effect of surface-active agents on gas hydrate nucleation by statistical measurements of phase-transitions. The different stages of hydrate growth will be investigated by confocal and transmission electron microscopes to understand how surface-active agents differently affect CH4 and CO2 hydrates in terms of hydrate crystal sizes and porosities.
Intellectual Merit: An important hypothesis to be tested is that the adsorption of surface-active molecules onto the hydrate surface rather than their micellization in bulk phase is the main reason for accelerating and inhibiting gas hydrate formation. The adsorption mechanisms to be studied as part of the proposed research will lead to a fundamental understanding of the role of surface-active agents in achieving different morphologies and formation rates of gas hydrates in terms of the microscopic observations. Novel neutron scattering studies of the microscopic, interfacial structure will clarify the configuration and functionality of surface-active agents at the oil-hydrate-water interface. This knowledge on hydrate morphologies and interfacial phenomena will be connected to the macroscopic, scale-up experiments of bulk-phase reaction.
Broader Impact: The fundamental understanding of the active role of surfactants and hydrate inhibitors enables controlled self-assembly of liquid-phase water molecules into solid gas hydrates. This understanding can be utilized for fast gas hydrate formation needed for safe natural gas storage and CO2 sequestration, while it can also be used for the investigation of an economical alternative to the 1 billion dollars per year spent on methanol as a hydrate inhibitor. This research will be performed by a close collaboration of two institutions, a Ph.D. granting school (CCNY) and an undergraduate liberal-arts college (Hamilton). The PIs will engage underrepresented high school students via existing programs and will actively involve undergraduate students and graduate students via a summer research exchange program between the two institutions. They will investigate gas hydrate-aided CO2 separation from flue gas to incorporate the thermodynamic, kinetic, interfacial science/engineering, and design aspects of this process into the undergraduate and graduate classes at Hamilton and CCNY.
This project is supported by two CBET programs: Interfacial Processes and Thermodynamics; Process and Reaction Engineering
Naturally occurring gas hydrates (methane hydrates) have drawn the public attention in recent years for their potential as a future energy resource. These gas hydrates are one class of clathrates that consist of host and guest molecules. The host forms a cavity by hydrogen-bonded water molecules where the guest, such as methane, carbon dioxide, cyclo-pentane, is enclathrated by the cavity. The synthetic gas hydrates provide a high density of gas storage and can be utilized as a safe and economical storage medium. On the other hand, the formation and agglomeration of hydrates inside crude oil and natural gas delivery lines imposes a nuisance upon the continuous production of oil and gas. Thus, both accelerating and retarding the gas hydrate formation are critical in methane recovery from the natural gas hydrate sediments, gas storage/CO2 sequestration, and prevention of hydrate clogging. Thus, the main objective of this research is to understand the effect of surface-active agents on hydrate formation from the molecular level to the bulk phase level. Intellectual merits: One hypotheses is surface-active molecules adsorb onto the hydrate surface rather than micellize in bulk phase. This has been verified as a main reason for accelerating and inhibiting gas hydrate formation using various analytical experimental techniques. We have determined the adsorption isotherms of surfactants and polymer inhibitors and have understood what adsorption patterns of the surface-active agents can lead to the different performance of gas hydrate formation/inhibition. This can be used for screening effective and economical surfactants and polymer inhibitors as for accelerating or retarding gas hydrate formation. Another important understanding is that surface-active agents will compete with bicarbonates or other anions for adsorption sites of hydrate particles if the gas phase contains CO2 or the aqueous phase is an electrolytic solution. Broader impacts: This project intensively involves in training several underrepresented high school and undergraduate/graduate students. They have been exposed to many aspects of surface science and engineering.The students with the research group designed many prototypes of low temperature and high-pressure cells for carrying out the experiments. The screening of effective gas hydrate kinetic inhibitors can lead to a dramatic cost reduction compared to the current practice of bulk-phase injection of high-dosage thermodynamic inhibitors. The understanding of hydrate formation under various surface-active agents provides a new way to accelerate or inhibit hydrate formation rates for methane recovery from methane hydrate sediments and CO2 sequestration in hydrate forms. The research results have been disseminated in more than 10 referred journals related to chemistry and chemical engineering and more than 10 presentations in the ACS and AIChE conferences. Outcomes: In the first year, we quantitatively determined the adsorption isotherm of the inhibitors using two different kinetic inhibitors (polyvinyl pyrrolidone (PVP) and polyvinylcaprolactam (PVCap)). The adsorption of PVP shows Langmuir-type adsorption while that of PVCap belongs to multi-layer type (BET-type) adsorption (Zhang et al., J. Phys. Chem. C, 2009). It was found that the PVCap multi-layer adsorption offers a thicker adsorption layer than that for the PVP case. This makes PVCap more effective in reducing the diffusion of hydrate formers from the bulk phase to the hydrate surface where the hydrate growth prefers to proceed. The competitive adsorption exists between carbonates and sodium dodecyl sulfate (SDS, anionic surfactant) on the tetrahydrofuran (THF) hydrates (Zhang et al., JCIS, 2010). The research group qualitatively analyzed the competitive adsorption of SDS and carbonates using pyrene fluorescence. The second year has produced fruitful results on the fundamental aspect of accelerated gas hydrate formation kinetics aided by a small amount of surfactant. We have found that the isotherms of two surfactants on cyclopentane (CP) hydrates, anionic sodium dodecyl sulfate (SDS) and cationic dodecyl-trimethylammonium bromide (DTAB), are of Langmuir-Step type which has two plateaus (Lo et al., J. Phys. Chem. C, 2010). In th first monolayer, tails orient parallel to each other and hydrate formers like methane molecules and hydrate-like water clusters could form micro-domains (Lo et al., J. Phys. Chem. Letters. 2010) at the interface as shown in Image 1. On the other hand, adsorption of DS- makes the hydrate surface more negative at higher SDS concentrations, which could make the water structure hydrate-like at the interface easier than the cationic DTAB case. In the third and final years, we have found that the type of SDS adsorption is still Langmuir-Step at NaCl concentrations up to 10 mM but the adsorbed amount of SDS is the highest at 1 mM NaCl (Salako et al., JCIS, 2012). We have understood from these salt experiments that 1 mM NaCl solution can help SDS adsorption because Cl- ions are not much competitive with DS- ions about adsorption sites due to the low NaCl concentration and the more SDS is adsorbed, the higher is methane hydrate formation rate.
