Wednesday, November 05, 2003
An Introdution to Natural Gas Hydrate Transportation
Hydrates were first studied thoroughly in the 1890’s.The detection of hydrates in gas pipelines in the 1930’s marked an important milestone to their impact in industry. After this event, many didn’t realize the potential of hydrates, and rather regarded them as a nuisance blocking the pipelines. However, in 1964, naturally occurring methane hydrate was discovered by a Russian drilling crew in Messoyakha, a Siberian gas field.
Interests in this subject deepened and scientists set out to find other locations around the world where methane could be found in the frozen form. In the 1970’s, methane hydrates were found in ocean sediments. In 1992, the Ocean Drilling Program began intentionally looking for hydrate deposits.They were able to bring up samples to the surface for further study.
The magnitude of the energy available for methane hydrates was initially quantified by the U.S. Geological Survey in 1995. The USGS suggested that hydrate deposits entrapped between 112,000 trillion cubic feet and 676,000 trillion cubic feet of methane. The estimate was refined in 1997 to a more conservative 200,000 trillion cubic feet. Even this lower estimate is significant when compared to the 1,400 trillion cubic feet in the nation's conventional gas reserves. On a world-wide basis, it is estimated that methane hydrate reserves are 400 million trillion cubic feet, compared with 5,000 trillion feet in known gas reserves.
[ Sun Oct 12, 09:53:53 PM | abm gheisi | edit ]
Hydrate Formation
The formation rate of natural gas hydrate is governed by a multitude of factors, including the pressure, temperature and gas composition, also called PVT-effects. Also, the rate of hydrate formation is determined by the combined effects of heat and mass transfer. Cooling is required to remove the hydrate heat of formation. Mass transport is required to dissolve the natural gas in liquid water, and to bring the dissolved gas molecules in contact with a growing hydrate crystal. In addition to the above factors, the rate of hydrate formation depends on the nature of crystal growth, also referred to as chemical reaction kinetics. The overall rate of hydrate formation therefore, depends on PVT-effects, transport-effects and reaction-effects. One way to gain knowledge about the overall rate of hydrate formation, is to carry out experiments at conditions similar to the conditions found in subsea production systems and process plants.
Gas hydrate formation is usually described as a crystallization process with nucleation, growth, agglomeration and breakage . Gas is dissolved in water, and nucleation starts primarily at the gas-water interface where the gas concentration is highest. The recently formed crystals disperse in the bulk liquid and start to grow as more gas is supplied to the liquid phase. Usually, after a while, agglomeration and breakage of the growing particles can be observed. In the literature, the gas consumption rate and the particle size distribution of the formed hydrate crystals are used to describe the hydrate formation process quantitatively.
In a model proposed by Vysniauskas and Bishnoi in 1983 , the rate of hydrate formation was modelled from the consumption rate of gas. Experiments were performed with methane and ethane gas and distilled water in a 0.5 litre semi-batch stirred reactor, and it was found that pressure, temperature driving force and gas-liquid interfacial area were the most important parameters affecting the gas consumption rate. Increased stirring rate and pressure resulted in increased consumption rate of gas, while an increase in temperature resulted in decreased consumption rate. The effect of stirring was due to the increased gas-liquid interfacial area with increased stirring rate.
In work done by Englezos et al. published in 1987, the gas consumption rate was coupled to the hydrate crystal growth rate. Experiments were carried out in the same equipment as used by Vysniauskas and Bishnoi . The growth was modelled using crystallization theory, and a two-film theory was adapted for the gas-liquid interfacial mass transfer. Experimental results were modelled so that the number of moles of gas consumed per particle per second was proportional to the fugacity driving force and the surface area of the particle. This fugacity driving force was defined as the difference in the fugacity of the dissolved gas and the three-phase equilibrium fugacity at the experimental temperature. The rate constants indicated a weak dependence on the temperature.
In 1994, Skovborg and Rasmussen simplified the Englezos-model by assuming that the rate of hydrate formation was mass-transport limited. In that way, the need for information about the crystal size distribution was eliminated. The experiments were performed in a semi-batch stirred reactor, and the gas consumption rate was modelled as a linear function of the difference in mole fraction of gas at the gas-liquid interface and the mole fraction in the liquid bulk phase. The rate constant was the product of the liquid side mass transfer coefficient and the total gas-liquid interfacial area in the reactor.
In 1999, Herri and co-workers published a comprehensive work based on both classical crystallization and mass transfer theory. The hydrate formation process was described by gas absorption, primary and secondary nucleation, growth, agglomeration and breakage. In a semi-batch stirred reactor of approximately 1 litre, consumption of methane gas and particle size distribution were measured with time. The influence of the stirring rate was measured and modelled, and found to be more complex than previously thought. In addition, the model predicts the development of the mean particle diameter and the total amount of particles.
An interesting study was published by Happel et al. in 1994 . Hydrate equilibrium conditions, hydrate formation and melting of methane hydrates and methane-nitrogen hydrates in a CSTR of 1 litre were studied. Methane gas consumption rates were presented but no model was proposed for the hydrate formation rate. However, the experimental formation rates were found to be much higher than the experimental results of Vysniauskas and Bishnoi. This led to the conclusion that applying results from batch reactors could prove difficult in design of a continuous hydrate formation reactor.
The above laboratory scale studies reveal the importance of knowing the effect of operational parameters on the gas consumption rate and the particle size distribution. However, as pointed out by Happel et al., new results and new models may be necessary to describe the rate of hydrate formation for large-scale continuous reactors
Hydrates were first studied thoroughly in the 1890’s.The detection of hydrates in gas pipelines in the 1930’s marked an important milestone to their impact in industry. After this event, many didn’t realize the potential of hydrates, and rather regarded them as a nuisance blocking the pipelines. However, in 1964, naturally occurring methane hydrate was discovered by a Russian drilling crew in Messoyakha, a Siberian gas field.
Interests in this subject deepened and scientists set out to find other locations around the world where methane could be found in the frozen form. In the 1970’s, methane hydrates were found in ocean sediments. In 1992, the Ocean Drilling Program began intentionally looking for hydrate deposits.They were able to bring up samples to the surface for further study.
The magnitude of the energy available for methane hydrates was initially quantified by the U.S. Geological Survey in 1995. The USGS suggested that hydrate deposits entrapped between 112,000 trillion cubic feet and 676,000 trillion cubic feet of methane. The estimate was refined in 1997 to a more conservative 200,000 trillion cubic feet. Even this lower estimate is significant when compared to the 1,400 trillion cubic feet in the nation's conventional gas reserves. On a world-wide basis, it is estimated that methane hydrate reserves are 400 million trillion cubic feet, compared with 5,000 trillion feet in known gas reserves.
[ Sun Oct 12, 09:53:53 PM | abm gheisi | edit ]
Hydrate Formation
The formation rate of natural gas hydrate is governed by a multitude of factors, including the pressure, temperature and gas composition, also called PVT-effects. Also, the rate of hydrate formation is determined by the combined effects of heat and mass transfer. Cooling is required to remove the hydrate heat of formation. Mass transport is required to dissolve the natural gas in liquid water, and to bring the dissolved gas molecules in contact with a growing hydrate crystal. In addition to the above factors, the rate of hydrate formation depends on the nature of crystal growth, also referred to as chemical reaction kinetics. The overall rate of hydrate formation therefore, depends on PVT-effects, transport-effects and reaction-effects. One way to gain knowledge about the overall rate of hydrate formation, is to carry out experiments at conditions similar to the conditions found in subsea production systems and process plants.
Gas hydrate formation is usually described as a crystallization process with nucleation, growth, agglomeration and breakage . Gas is dissolved in water, and nucleation starts primarily at the gas-water interface where the gas concentration is highest. The recently formed crystals disperse in the bulk liquid and start to grow as more gas is supplied to the liquid phase. Usually, after a while, agglomeration and breakage of the growing particles can be observed. In the literature, the gas consumption rate and the particle size distribution of the formed hydrate crystals are used to describe the hydrate formation process quantitatively.
In a model proposed by Vysniauskas and Bishnoi in 1983 , the rate of hydrate formation was modelled from the consumption rate of gas. Experiments were performed with methane and ethane gas and distilled water in a 0.5 litre semi-batch stirred reactor, and it was found that pressure, temperature driving force and gas-liquid interfacial area were the most important parameters affecting the gas consumption rate. Increased stirring rate and pressure resulted in increased consumption rate of gas, while an increase in temperature resulted in decreased consumption rate. The effect of stirring was due to the increased gas-liquid interfacial area with increased stirring rate.
In work done by Englezos et al. published in 1987, the gas consumption rate was coupled to the hydrate crystal growth rate. Experiments were carried out in the same equipment as used by Vysniauskas and Bishnoi . The growth was modelled using crystallization theory, and a two-film theory was adapted for the gas-liquid interfacial mass transfer. Experimental results were modelled so that the number of moles of gas consumed per particle per second was proportional to the fugacity driving force and the surface area of the particle. This fugacity driving force was defined as the difference in the fugacity of the dissolved gas and the three-phase equilibrium fugacity at the experimental temperature. The rate constants indicated a weak dependence on the temperature.
In 1994, Skovborg and Rasmussen simplified the Englezos-model by assuming that the rate of hydrate formation was mass-transport limited. In that way, the need for information about the crystal size distribution was eliminated. The experiments were performed in a semi-batch stirred reactor, and the gas consumption rate was modelled as a linear function of the difference in mole fraction of gas at the gas-liquid interface and the mole fraction in the liquid bulk phase. The rate constant was the product of the liquid side mass transfer coefficient and the total gas-liquid interfacial area in the reactor.
In 1999, Herri and co-workers published a comprehensive work based on both classical crystallization and mass transfer theory. The hydrate formation process was described by gas absorption, primary and secondary nucleation, growth, agglomeration and breakage. In a semi-batch stirred reactor of approximately 1 litre, consumption of methane gas and particle size distribution were measured with time. The influence of the stirring rate was measured and modelled, and found to be more complex than previously thought. In addition, the model predicts the development of the mean particle diameter and the total amount of particles.
An interesting study was published by Happel et al. in 1994 . Hydrate equilibrium conditions, hydrate formation and melting of methane hydrates and methane-nitrogen hydrates in a CSTR of 1 litre were studied. Methane gas consumption rates were presented but no model was proposed for the hydrate formation rate. However, the experimental formation rates were found to be much higher than the experimental results of Vysniauskas and Bishnoi. This led to the conclusion that applying results from batch reactors could prove difficult in design of a continuous hydrate formation reactor.
The above laboratory scale studies reveal the importance of knowing the effect of operational parameters on the gas consumption rate and the particle size distribution. However, as pointed out by Happel et al., new results and new models may be necessary to describe the rate of hydrate formation for large-scale continuous reactors
