68
3 Gas Hydrates
the liquid guest phase as merely “oil” and the liquid water phase as merely “water”
(strictly speaking, an “oil” phase is an oil-rich phase that is saturated with water and
a “water” phase is a water-rich phase that is saturated with the guest).
Three triple-phase curves on which three phases coexist meet at each quadruple
point. The areas between the triple-phase curves are two-phase regions. The identity
of the two phases in each such region is defined by the two phases that are common
to the adjacent triple-phase curves. For example, the lower left region bounded by the
two triple-phase curves of ice–clathrate–gas and ice–water–gas consists of ice and
gas. Likewise, the large lower right region bounded by the three triple-phase curves
of oil–water–gas, water–clathrate–gas, and ice–water–gas consists of water and gas.
The physical meaning of Fig. 3.2 is as follows. We first consider isobaric processes.
At an initial pressure lower than the lower quadruple point and at a high temperature,
the system initially consists of two phases of liquid water and gas. As the system
cools isobarically, the system reaches the triple-phase curve of ice–water–gas (i.e.,
ice newly forms). Below this temperature, the system consists of two phases of
ice and gas (liquid water disappears). At an initial pressure between that of the
upper quadruple point and that of the lower quadruple point, the system initially
consists of two phases of liquid water and gas at a high temperature. As the system
cools isobarically, the system reaches the triple-phase curve of water–clathrate–gas
(clathrate newly forms). Below this temperature, the system consists of two phases
of clathrate and gas (liquid water disappears). At an initial pressure higher than the
upper quadruple point, the system initially consists of two phases of liquid water and
gas at a very high temperature. As the system cools isobarically, the system reaches
the triple-phase curve of oil–water–gas (oil newly forms). Below this temperature, the
gas liquefies and the system consists of two phases of oil and water (gas disappears).
As the system further cools, the system reaches another triple-phase curve of oil–
water–clathrate (clathrate newly forms). Below this temperature, all the water in the
system is consumed for the formation of clathrate, and the system consists of two
phases of the guest oil, which is supposed to exist in excess of water, and the clathrate.
Similar considerations can be applied to isothermal processes. At an initial temperature that is lower than the lower quadruple point and a low pressure, the system
initially consists of two phases of ice and gas. As the system is pressurized isothermally, the system reaches the triple-phase curve of ice–clathrate–gas (clathrate newly
forms). Above this pressure, all the ice in the system is consumed for the formation
of clathrate, and the system consists of two phases of guest gas, which is supposed to
exist in excess of water, and the clathrate. As the pressure is further increased isothermally, the system reaches another triple-phase curve of oil–clathrate–gas (oil newly
forms). Above this pressure, all the excess gas is liquefied, and the system consists
of two phases of an excess oil and the clathrate. At an initial temperature between
that of the upper quadruple point and that of the triple-phase curve of ice–water–gas,
the system initially consists of two phases of liquid water and gas at a low pressure.
As the system is pressurized isothermally, the system reaches the triple-phase curve
of water–clathrate–gas (clathrate newly forms). Above this pressure, all the water
in the system is consumed for the formation of clathrate, and the system consists of
two phases of an excess gas and the clathrate. As the pressure is further increased
3 Gas Hydrates
the liquid guest phase as merely “oil” and the liquid water phase as merely “water”
(strictly speaking, an “oil” phase is an oil-rich phase that is saturated with water and
a “water” phase is a water-rich phase that is saturated with the guest).
Three triple-phase curves on which three phases coexist meet at each quadruple
point. The areas between the triple-phase curves are two-phase regions. The identity
of the two phases in each such region is defined by the two phases that are common
to the adjacent triple-phase curves. For example, the lower left region bounded by the
two triple-phase curves of ice–clathrate–gas and ice–water–gas consists of ice and
gas. Likewise, the large lower right region bounded by the three triple-phase curves
of oil–water–gas, water–clathrate–gas, and ice–water–gas consists of water and gas.
The physical meaning of Fig. 3.2 is as follows. We first consider isobaric processes.
At an initial pressure lower than the lower quadruple point and at a high temperature,
the system initially consists of two phases of liquid water and gas. As the system
cools isobarically, the system reaches the triple-phase curve of ice–water–gas (i.e.,
ice newly forms). Below this temperature, the system consists of two phases of
ice and gas (liquid water disappears). At an initial pressure between that of the
upper quadruple point and that of the lower quadruple point, the system initially
consists of two phases of liquid water and gas at a high temperature. As the system
cools isobarically, the system reaches the triple-phase curve of water–clathrate–gas
(clathrate newly forms). Below this temperature, the system consists of two phases
of clathrate and gas (liquid water disappears). At an initial pressure higher than the
upper quadruple point, the system initially consists of two phases of liquid water and
gas at a very high temperature. As the system cools isobarically, the system reaches
the triple-phase curve of oil–water–gas (oil newly forms). Below this temperature, the
gas liquefies and the system consists of two phases of oil and water (gas disappears).
As the system further cools, the system reaches another triple-phase curve of oil–
water–clathrate (clathrate newly forms). Below this temperature, all the water in the
system is consumed for the formation of clathrate, and the system consists of two
phases of the guest oil, which is supposed to exist in excess of water, and the clathrate.
Similar considerations can be applied to isothermal processes. At an initial temperature that is lower than the lower quadruple point and a low pressure, the system
initially consists of two phases of ice and gas. As the system is pressurized isothermally, the system reaches the triple-phase curve of ice–clathrate–gas (clathrate newly
forms). Above this pressure, all the ice in the system is consumed for the formation
of clathrate, and the system consists of two phases of guest gas, which is supposed to
exist in excess of water, and the clathrate. As the pressure is further increased isothermally, the system reaches another triple-phase curve of oil–clathrate–gas (oil newly
forms). Above this pressure, all the excess gas is liquefied, and the system consists
of two phases of an excess oil and the clathrate. At an initial temperature between
that of the upper quadruple point and that of the triple-phase curve of ice–water–gas,
the system initially consists of two phases of liquid water and gas at a low pressure.
As the system is pressurized isothermally, the system reaches the triple-phase curve
of water–clathrate–gas (clathrate newly forms). Above this pressure, all the water
in the system is consumed for the formation of clathrate, and the system consists of
two phases of an excess gas and the clathrate. As the pressure is further increased