342
Water for Energy and Fuel Production
dew point (i.e., must be water saturated). In addition, dehydration can be used as
an alternative. An addition of an inhibitor can shift pressure–temperature diagram
such that the temperature decreases at specific pressures, and this facilitates hydrate
dissociation. After temperature depression due to an addition of an inhibitor, free
gas will form and hydrate zone will shift to the left to lower the temperature side.
Methanol has a high vapor pressure and infinite water solubility and can easily shift
to the gas phase.
In most offshore applications, hydrate formation is controlled by injection of a
thermodynamic hydrate inhibitor. Inhibitor injection at a given pressure will reduce
the temperature at which hydrate is formed. Overall, ethylene glycol seemed to be
the most useful inhibitor for the gas hydrates.
12.5.4 gAS exChAnge
Exchanging CO 2 with CH 4 concept was first advanced by Ohgaki et al. [68]. Their
experimental study showed that CO 2 be preferentially clathrated over CH 4 in the
hydrated phase. They also demonstrated the possibility of producing CH 4 by injecting CO 2 gas. Ohgaki et al. [68] noted that during the exchange process, mole fraction
of CO 2 in the hydrate phase was greater than that in the gas phase.
This effect was further studied quantitatively by Seo and Lee [69] and Seo et al.
[70]. They showed that CO 2 concentration in the hydrate phase was >90% when gasphase concentration of CO 2 in the hydrate formers (i.e., CO 2 and CH 4 ) was above
40%. Pure CH 4 and CO 2 form structure I (sI) type hydrates, and their mixtures also
form sI type hydrates [61–75]. In forming mixed CH 4 and CO 2 hydrates, the CH 4
molecules occupy both the large and small cages of sI type hydrates, whereas CO 2
molecules only occupy the large cages. Without hydrate dissociation, there is an
upper limit to the substitution of CO 2 for CH 4 in hydrates.
Lee et al. [218] showed that ~64% of CH 4 can be released by exchange with
CO 2 . In addition to equilibrium considerations, the heat of CO 2 hydrate formation
is higher (−57.9 kJ/mol) than the heat of dissociation of CH 4 hydrate (−54.5 kJ/mol),
making the overall process exothermic that favors the normal exchange of CO 2 with
CH 4 hydrate.
While the exchange of CO 2 for CH 4 is thermodynamically a favorable process, the kinetics of exchange mechanism is slow [61–75,209], with induction time
requiring several days. The original studies also did not address the rate of CO 2
gas penetration further into gas hydrate, beyond the first few hundred manometers
at the interface [203]. The exchange of CO 2 with CH 4 at high pressure (with liquid
CO 2 ) was also examined in the literature, but once again slow rate of exchange was
observed. The use of nitrogen instead of CO 2 gave a much higher rate. For liquid
CO 2 injection, thermodynamic conditions can either favor CO 2 or CH 4 cage occupation [76–83]. This transition occurs when the pure CO 2 and CH 4 temperatureversus-pressure equilibrium functions cross at the pressure above the gas–liquid
CO 2 phase boundary.
Thermodynamic properties of hydrates depend on the pore size distribution in
the geologic media; hydrate formation will occur in large pores first and then in
small pores until equilibrium is achieved [205,212]. Porous media also affect other
Water for Energy and Fuel Production
dew point (i.e., must be water saturated). In addition, dehydration can be used as
an alternative. An addition of an inhibitor can shift pressure–temperature diagram
such that the temperature decreases at specific pressures, and this facilitates hydrate
dissociation. After temperature depression due to an addition of an inhibitor, free
gas will form and hydrate zone will shift to the left to lower the temperature side.
Methanol has a high vapor pressure and infinite water solubility and can easily shift
to the gas phase.
In most offshore applications, hydrate formation is controlled by injection of a
thermodynamic hydrate inhibitor. Inhibitor injection at a given pressure will reduce
the temperature at which hydrate is formed. Overall, ethylene glycol seemed to be
the most useful inhibitor for the gas hydrates.
12.5.4 gAS exChAnge
Exchanging CO 2 with CH 4 concept was first advanced by Ohgaki et al. [68]. Their
experimental study showed that CO 2 be preferentially clathrated over CH 4 in the
hydrated phase. They also demonstrated the possibility of producing CH 4 by injecting CO 2 gas. Ohgaki et al. [68] noted that during the exchange process, mole fraction
of CO 2 in the hydrate phase was greater than that in the gas phase.
This effect was further studied quantitatively by Seo and Lee [69] and Seo et al.
[70]. They showed that CO 2 concentration in the hydrate phase was >90% when gasphase concentration of CO 2 in the hydrate formers (i.e., CO 2 and CH 4 ) was above
40%. Pure CH 4 and CO 2 form structure I (sI) type hydrates, and their mixtures also
form sI type hydrates [61–75]. In forming mixed CH 4 and CO 2 hydrates, the CH 4
molecules occupy both the large and small cages of sI type hydrates, whereas CO 2
molecules only occupy the large cages. Without hydrate dissociation, there is an
upper limit to the substitution of CO 2 for CH 4 in hydrates.
Lee et al. [218] showed that ~64% of CH 4 can be released by exchange with
CO 2 . In addition to equilibrium considerations, the heat of CO 2 hydrate formation
is higher (−57.9 kJ/mol) than the heat of dissociation of CH 4 hydrate (−54.5 kJ/mol),
making the overall process exothermic that favors the normal exchange of CO 2 with
CH 4 hydrate.
While the exchange of CO 2 for CH 4 is thermodynamically a favorable process, the kinetics of exchange mechanism is slow [61–75,209], with induction time
requiring several days. The original studies also did not address the rate of CO 2
gas penetration further into gas hydrate, beyond the first few hundred manometers
at the interface [203]. The exchange of CO 2 with CH 4 at high pressure (with liquid
CO 2 ) was also examined in the literature, but once again slow rate of exchange was
observed. The use of nitrogen instead of CO 2 gave a much higher rate. For liquid
CO 2 injection, thermodynamic conditions can either favor CO 2 or CH 4 cage occupation [76–83]. This transition occurs when the pure CO 2 and CH 4 temperatureversus-pressure equilibrium functions cross at the pressure above the gas–liquid
CO 2 phase boundary.
Thermodynamic properties of hydrates depend on the pore size distribution in
the geologic media; hydrate formation will occur in large pores first and then in
small pores until equilibrium is achieved [205,212]. Porous media also affect other