Gas hydrates are known to form plugs in pipelines. Hydrate plug dissociation times can be predicted using the CSMPlug program. At high methane mole fractions of a methane + ethane mixture the predictions agree with experiments for the relative dissociation times of structure I (sI) and structure II (sII) plugs. At intermediate methane mole fractions the predictions disagree with experiment. Enthalpies of dissociation were measured and predicted with the Clapeyron equation. The enthalpies of dissociation for the methane + ethane hydrates were found to vary significantly with pressure, the composition, and the structure of hydrate. The prediction and experimental would likely agree if this variation in the enthalpy of dissociation was taken in to account. In doing the plug dissociation studies at high methane mole fraction a discontinuity was observed in the gas evolution rate and X-ray diffraction indicated the possibility of the presence of both sI and sII hydrate structures. A detailed analysis by step-wise modelling utilising the hydrate prediction package CSMGem showed that preferential enclathration could occur. This conclusion was supported by experiment. Salts such as tetraisopentylammonium fluoride form semi-clathrate hydrates with melting points higher than 30 ℃ and vacant cavities that can store cages such as methane and hydrogen. The stability of this semi-clathrate hydrate with methane was studied and the dissociation phase boundary was found to be at temperatures of about (25 to 30) K higher than that of methane hydrate at the same pressure.

Gas hydrates are a topic of great interest and intense investigation. Traditionally, these compounds have been seen as a nuisance to the oil and gas industry, which can plug pipelines and cause hours of costly downtime. More recently, gas hydrates have been viewed as a possible energy source due to the vast amount of methane trapped in the form of gas hydrate. Many researchers have also proposed the possibility of transporting natural gas in the form of gas hydrate may be safer and more economical than using liquid or compressed natural gas. Gas hydrate may also offer the possibility of reducing greenhouse gas emissions via the sequestration of carbon dioxide. / Surfactants have been found to act as both promoters and inhibitors of hydrate formation. In the present study, the formation rate, solubility and mass transfer conductance of methane in the presence of Gemini surfactant, a new class of surfactants, was studied with varying concentration of Gemini surfactant. The experiments to determine the formation rates of methane hydrate were conducted at 4°C and 6500 kPa. While the experiments to determine solubility and mass conductance were carried out at 4°C and 3800 kPa. The resulting values were used to determine experimental accuracy and reproducibility by comparing the values obtained with literature values and by analyzing the distribution of the data obtained. Solubility measurements were extremely close to literature values with only a 1.4% difference. The distribution of solubility values and formation rates did not deviate significantly between replicates indicating a high degree of reproducibility; however, a lot of variability was observed in mass transfer conductance. This may be attributed to the fact that mass transfer was not determined experimentally by regressing a coefficient to fit a curve, which may be less accurate than other experimentally determined parameters. / In the second part of the study, the formation rate, solubility and mass transfer conductance of methane were determined using aqueous Gemini surfactant solutions. The experiments to determine the formation rates of methane hydrate were conducted at 4°C and 6500 kPa. While the experiments to determine solubility and mass transfer conductance were carried out at 4°C and 3800 kPa. The resulting values were used to determine the effect of Gemini surfactant on the properties of interest by comparing the values obtained with aqueous Gemini surfactant with the values previously obtained for pure water. The results obtained showed that solubility increased with increasing concentrations of Gemini surfactant with solubility increasing by up to 18% for higher concentration of Gemini surfactant. The mass transfer conductance was also found to increase by up to 49%; however other than the existence of an increase, no conclusive relationship could be determined between the concentration of Gemini surfactant and mass transfer conductance. / Finally, the formation rate of gas hydrates was found to decrease slightly, relative to water, at low concentrations, increased linearly at subsequently higher concentrations and ultimately plateau at a maximum. This trend was in agreement with similar experiments found in literature and the increase in formation rate may be attributed to the increase in both solubility and mass transfer conductance when using aqueous Gemini surfactant.

Carbon dioxide CO₂capture through hydrate formation is a novel technology under consideration as an efficient means of separating CO₂from flue/fuel gas mixtures for sequestration and enhanced oil recovery operations. This thesis examines post-combustion capture of CO₂from fossil-fuel power plant flue-gas streams through hydrate formation in a silica gel column. Power plant flue-gas contains essentially CO₂and nitrogen (N2) after suitable pre-treatment steps, thus a model flue-gas comprising 17% co₂and 83% N2 was used in the study. Previous studies employed a stirred-tank reactor to achieve water-gas contact for formation of hydrates; recent microscopic studies involved using water dispersed in silica gel to react with gas, showing potential for improved hydrate formation rates without the need for agitation. This study focuses on macroscopic kinetics of hydrate formation in silica gel to evaluate hydrate formation rates, CO₂separation efficiency and determining optimal silica gel properties as a basis for a CO2 capture process. Spherical silica gels with 30.0 and 100.0 nm pore sizes and 40-75 and 75-200 μm particle sizes were studied to determine pore size and particle size effects on hydrate formation. 100.0 nm pores achieved higher gas uptake and CO₂recovery over the 30.0 nm case. Improved CO₂separation was obtained when 75-200 μm particles with 100.0 nm pores were used. The two effects observed are due to improved gas diffusion occurring with larger pore and particle size, favouring increased hydrate formation. Compared to stirred-tank experiments, results in this study show a near four-fold increase in moles of gas incorporated in the hydrate per mole of water, showing that improved water-to-hydrate conversion is obtained with pore-dispersed water. At similar experimental conditions, CO₂recovery improved from 42% for stirred-tank studies to 51% for the optimum silica (100.0 nm 75-200 μm) determined in this study. Finally, effects of tetrahydrofuran (THF) - an additive that reduces operating pressure were evaluated. Experiments with 1 mol% THF, the optimum determined from previous stirred tank studies, showed improved gas consumption in silica but reduced CO₂recovery, indicating that the optimum concentration for use in silica is different from that in stirred-tank experiments.

Thesis (M.S.)--Mississippi State University. Dave C. Swalm School of Chemical Engineering. / Title from title screen. Includes bibliographical references.

The class of water-ice compound known as gas hydrate has been of interest to science for sometime where, for instance, gas hydrates make excellent candidates for studying the interactions of water and gas molecules. They are also of relevance to industry, where they present an interesting material for the separation, transport, and storage of different gases, and also due to the vast quantities of methane gas that are trapped in natural gas hydrate formations. While much is known about the behaviour of many gas hydrate systems at high-pressure, the CO2 hydrate system is less well studied, with apparent hydrate dissociation at just 10 kbar, and (prior to this work) an unsolved crystalline phase in the pressure range 6-10 kbar. In this work the CO2-H2O system has been studied at high-pressure and, by heating samples to the liquid state and observing their behaviour on refreezing, it has been confirmed that there are indeed no hydrate phases in the system above 10 kbar (up to at least 40 kbar). While performing this investigation, an interesting effect of CO2 on the behaviour of water crystallisation was also observed, and additionally, a simple yet effective technique for making solubility measurements in the system at high-pressure has been discovered. Using a combination of neutron and x-ray diffraction techniques, the crystal structure of the previously unsolved ‘HP’ CO2 hydrate phase has been determined by ab-initio methods. It has been found to be a new gas hydrate structure, but is shared by a small number of Zintl compounds, and may also be common to the unsolved C0 phase of H2 hydrate. The structure has a characteristic spiral of guest molecule sites, leading to its suggested label as the spiral hydrate structure (s-Sp). Its composition has been measured as a tri-hydrate, and the compressibility of s-Sp and the low-pressure s-I CO2 hydrate phases have also been measured. On cooling to 77 K it has been discovered that a third CO2 hydrate phase is formed with a significantly larger unit cell, which is thought to possess a structure similar to that of s-Sp, but with an ordered arrangement of CO2 molecules. Finally, a pilot study of the high-pressure behaviour of the binary H2-CO2 hydrate system has been performed. Using Raman spectroscopy it has been found that a new mixed hydrate phase exists in the pressure range 5-15 kbar, and it is speculated that this could exhibit a freely tunable H2/CO2 content, based on suspicion that it forms the s-Sp structure. Additionally, it has been found that H2 and CO2 chemically react at room temperature, when compressed to ~5 kbar in a rhenium gasket. From the Raman spectrum this reaction product has been identified to be aqueous-methanol.

This thesis studies methane hydrate crystal formation and decomposition at 1.0, 4.0 and 7.0°C in a new apparatus. Hydrate was formed in the interstitial space of a variable volume bed of silica sand particles with an average diameter equal to 329μm (150 to 630μm range). The initial pressure inside the reactor was 8.0MPa for all the formation experiments. Three bed sizes were employed in order to observe the effects of the silica sand bed size on the rate of methane consumption (formation) and release (decomposition). The temperature at various locations inside the silica sand bed was measured with thermocouples during formation and decomposition experiments. For the decomposition experiments, two different methods were employed to dissociate the hydrate: thermal stimulation and depressurization. It was found that more than 74.0% of water conversion to hydrates was achieved in all hydrate formation experiments at 4.0°C and 1.0°C starting with a pressure of 8.0MPa. The dissociation of hydrate was found to occur in two stages when thermal stimulation was employed whereas three stages were found during depressurization. In both cases, the first stage was strongly affected by the changing bed size whereas it was not found to depend on the bed size afterwards. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

Carbon dioxide CO₂capture through hydrate formation is a novel technology under consideration as an efficient means of separating CO₂from flue/fuel gas mixtures for sequestration and enhanced oil recovery operations. This thesis examines post-combustion capture of CO₂from fossil-fuel power plant flue-gas streams through hydrate formation in a silica gel column. Power plant flue-gas contains essentially CO₂and nitrogen (N2) after suitable pre-treatment steps, thus a model flue-gas comprising 17% co₂and 83% N2 was used in the study. Previous studies employed a stirred-tank reactor to achieve water-gas contact for formation of hydrates; recent microscopic studies involved using water dispersed in silica gel to react with gas, showing potential for improved hydrate formation rates without the need for agitation. This study focuses on macroscopic kinetics of hydrate formation in silica gel to evaluate hydrate formation rates, CO₂separation efficiency and determining optimal silica gel properties as a basis for a CO2 capture process. Spherical silica gels with 30.0 and 100.0 nm pore sizes and 40-75 and 75-200 μm particle sizes were studied to determine pore size and particle size effects on hydrate formation. 100.0 nm pores achieved higher gas uptake and CO₂recovery over the 30.0 nm case. Improved CO₂separation was obtained when 75-200 μm particles with 100.0 nm pores were used. The two effects observed are due to improved gas diffusion occurring with larger pore and particle size, favouring increased hydrate formation. Compared to stirred-tank experiments, results in this study show a near four-fold increase in moles of gas incorporated in the hydrate per mole of water, showing that improved water-to-hydrate conversion is obtained with pore-dispersed water. At similar experimental conditions, CO₂recovery improved from 42% for stirred-tank studies to 51% for the optimum silica (100.0 nm 75-200 μm) determined in this study. Finally, effects of tetrahydrofuran (THF) - an additive that reduces operating pressure were evaluated. Experiments with 1 mol% THF, the optimum determined from previous stirred tank studies, showed improved gas consumption in silica but reduced CO₂recovery, indicating that the optimum concentration for use in silica is different from that in stirred-tank experiments. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

Methane hydrates have drawn the attention of climate scientists in the past decades due to the potency of methane as a greenhouse gas and the widespread occurrence of hydrates both in terrestrial and marine environments, which, if destabilised, could enhance global warming. This study aims to investigate how much impact sediment type has on modeled methane escape at the feather edge of stability for methane hydrates in the high latitude Northern Hemisphere (45° to 75° N). This area is characterised by cool bottom-water temperatures leading to a shallow gas hydrate stability zone (GHSZ), and has been disproportionally influenced by contemporary seawater warming. Calculations were performed to establish the depths of the upper and lower boundaries of the feather edge of the GHSZ. These limits were used to estimate seafloor areas covered by three select sediment types that have different petrophysical properties - hemipelagic clay, calcareous ooze and siliceous ooze. Modeling of methane flux for 300 years following a 3°C warming during the first 100 years was performed using TOUGH + HYDRATE for each of the three sediment types. The sediments behaved significantly differently, with siliceous ooze releasing the most methane gas, and calcareous ooze releasing the least. Estimates of total methane gas release were also performed on the areas covered by the three sediments between latitudes 45° to 75° N, and showed that, over the course of 300 years, up to 5 times the current methane concentration in the atmosphere could become susceptible to leaving methane hydrate reservoirs.

No description available.

An estimated 1000 trillion cubic meters of gas in the unconventional hydrocarbon resource of gas hydrates in the world?s ocean floors far exceeds the known hydrocarbons in conventional reserves like coal, petroleum, and natural gas. These hydrate deposits also contain massive amounts of the greenhouse gases like methane and carbon dioxide. As relatively little is known about the oceanloor natural gas hydrates, mechanisms leading to the formation of these hydrates in ocean sediments need to be investigated before the significant technical challenges of recovery and environmental hazards are addressed. The subject research focuses on possible catalytic effects of biosurfactants on the formation of natural gas hydrates in oceanloor sediments. Sand/clay packs were saturated with seawater containing 1000 ppm of biosurfactant and pressurized with natural gas of 90 mole% methane, 6 mole% ethane and 4 mole% propane. The experimental results showed that gas hydrates formation in porous media is catalyzed by biosurfactants at very low concentrations. Commercially available representatives from the five biosurfactant classifications that microbes produce were purchased and evaluated in sand/clay packs at hydrateorming conditions. The rate of formation and induction time differed in the presence of bentonite and kaolin. The surface activities of biosurfactants were either specific to sand or clay surfaces. While in the presence of bentonite, Surfactin decreased hydrate induction time by 71% over a reference test with no biosurfactant in the seawater; Surfactin lowered induction time by 25% in the presence of kaolin. Rhamnolipid reduced the induction time by 58% in the presence of bentonite and by 66% in the presence of kaolin. Snomax and Emulsan, belonging to the classification of polysaccharide lipid complexes, reduced induction time by 30 to 40% in the presence of both kaolin and bentonite. Fatty acids reduced the induction time by 55% in the presence of bentonite and by 20% in the presence of kaolin. Surfactin enhanced the rate of formation by 400% in the presence of bentonite, but it had minimal effect in the presence of kaolin. Emulsan and Snomax increased the rate of formation by 250%, while rhamnolipid and phospholipids doubled formation rate in the presence of bentonite. Emulsan increased the rate of formation by 800%. In seawater, at hydrateorming conditions, rhamnolipid was found to have a critical micellar concentration of 12 ppm. This very low value of CMC suggests that minimal bacterial activity in ocean sediments could greatly catalyze hydrate formation. The recent analysis by Lanoil et al. (2001) of sediments from around gas hydrate mounds in the Gulf of Mexico gives a direct association between microbes and gas hydrates and supports the conclusions of the subject work.