3.2 Thermodynamic Aspects of Gas Hydrates
73
The solubility of a guest gas in liquid water that can coexist with its clathrate
hydrate for an excess water composition is an important parameter that aids calculations of the chemical potential of the guest in the clathrate hydrate. Likewise, the
vapor pressure of water in the atmosphere of the guest gas that can coexist with
its clathrate hydrate for an excess guest composition is an important parameter that
aids calculations of the chemical potential of the host in the clathrate hydrate. Yet,
very few relevant data have been reported [1]: (1) methane and ethane solubility in
water as a function of temperature ramping rate [67], (2) carbon dioxide solubility
in water [68], (3) methane in water in the (liquid water–clathrate hydrate) two-phase
region [69], and (4) the aqueous methane solubility [70]. Qualitatively, the solubility
of a given guest in liquid water should decrease still further from the already low
value when the clathrate hydrate phase coexists because the activity of water should
decrease to match the lower chemical potential of water in the clathrate phase. Likewise, the vapor pressure of the water vapor over a clathrate hydrate in the atmosphere
of a guest gas should decrease further from that over liquid water.
3.3 Gas Hydrate Inhibitors
An industrially important issue that concerns clathrate hydrates is flow assurance
of oil and natural gas pipelines and flow lines. Production fluids are rarely dry, and
the water content generally increases as the field ages. The presence of water in a
high-pressure and low-temperature environment is prone to clathrate hydrate formation. Thus, a pipeline that travels low-temperature sections poses risks of undesired
clathrate hydrate formation that could cause blockage. A clathrate hydrate plug will
induce a dangerous pressure differential between the upstream and the downstream
sides of the plug which, if not remediated in a timely manner, could cause a major
accident. Oil and gas operators have been heavily using chemicals (gas hydrate
inhibitors) to prevent such adverse incident from occurring. There are three classes
of gas hydrate inhibitors: Thermodynamic hydrate inhibitors (THI), kinetic hydrate
inhibitors (KHI), and anti-agglomerates (AA). Since studies on the nucleation of
clathrate hydrates often involve gas hydrate inhibitors, we briefly describe each of
these groups of chemicals and anti-freeze proteins.
3.3.1 Thermodynamic Hydrate Inhibitors (THI)
Thermodynamic inhibition is colligative in nature. A THI is miscible with water
at all proportions and lowers the activity of water. Among the possible solutes for
thermodynamic inhibition, salts are usually not preferred due to their adverse corrosive effects on production facilities. Rather, alcohols and glycols are the preferred
THIs. In either case, a greater free energy reduction is required for clathrate hydrate
73
The solubility of a guest gas in liquid water that can coexist with its clathrate
hydrate for an excess water composition is an important parameter that aids calculations of the chemical potential of the guest in the clathrate hydrate. Likewise, the
vapor pressure of water in the atmosphere of the guest gas that can coexist with
its clathrate hydrate for an excess guest composition is an important parameter that
aids calculations of the chemical potential of the host in the clathrate hydrate. Yet,
very few relevant data have been reported [1]: (1) methane and ethane solubility in
water as a function of temperature ramping rate [67], (2) carbon dioxide solubility
in water [68], (3) methane in water in the (liquid water–clathrate hydrate) two-phase
region [69], and (4) the aqueous methane solubility [70]. Qualitatively, the solubility
of a given guest in liquid water should decrease still further from the already low
value when the clathrate hydrate phase coexists because the activity of water should
decrease to match the lower chemical potential of water in the clathrate phase. Likewise, the vapor pressure of the water vapor over a clathrate hydrate in the atmosphere
of a guest gas should decrease further from that over liquid water.
3.3 Gas Hydrate Inhibitors
An industrially important issue that concerns clathrate hydrates is flow assurance
of oil and natural gas pipelines and flow lines. Production fluids are rarely dry, and
the water content generally increases as the field ages. The presence of water in a
high-pressure and low-temperature environment is prone to clathrate hydrate formation. Thus, a pipeline that travels low-temperature sections poses risks of undesired
clathrate hydrate formation that could cause blockage. A clathrate hydrate plug will
induce a dangerous pressure differential between the upstream and the downstream
sides of the plug which, if not remediated in a timely manner, could cause a major
accident. Oil and gas operators have been heavily using chemicals (gas hydrate
inhibitors) to prevent such adverse incident from occurring. There are three classes
of gas hydrate inhibitors: Thermodynamic hydrate inhibitors (THI), kinetic hydrate
inhibitors (KHI), and anti-agglomerates (AA). Since studies on the nucleation of
clathrate hydrates often involve gas hydrate inhibitors, we briefly describe each of
these groups of chemicals and anti-freeze proteins.
3.3.1 Thermodynamic Hydrate Inhibitors (THI)
Thermodynamic inhibition is colligative in nature. A THI is miscible with water
at all proportions and lowers the activity of water. Among the possible solutes for
thermodynamic inhibition, salts are usually not preferred due to their adverse corrosive effects on production facilities. Rather, alcohols and glycols are the preferred
THIs. In either case, a greater free energy reduction is required for clathrate hydrate