The main purpose of this study was to develop a new
hydrate-based gas separation (HBGS) process especially
for recovering CO2 from flue gas. Temperature and pressure
conditions for hydrate formation have been closely
examined at the various CO2 concentrations of flue gases.
Tetrahydrofuran (THF) chosen as a hydrate promoter
can also participate in forming hydrates and produces a
mixed hydrate together with CO2. The hydrate stability region
was greatly expanded by using THF for lowering the
equilibrium formation pressure. To confirm thermodynamic
validity of the HBGS process, the three-phase equilibria
of hydrate, liquid, and vapor were measured for the systems
comprising CO2, N2 and water with or without THF in the
temperature range of 272−295 K. In addition, two phase
equilibria of hydrate and vapor were experimentally
investigated for the same systems at several temperatures.
Through close examination of the overall experimental
results, it was firmly verified that the HBGS process makes
it possible to recover more than 99 mol % of CO2 from
the flue gas. The key unit operations of the HBGS lie in
hydrate formation and subsequent dissociation similarly to
gas absorption and desorption using the sterically
hindered amines. The HBGS provides several advantages
over the conventional ones. First, the operational
temperature is moderate in the range of 273−283 K, and
continuous operation allows this process to treat a large
amount of gaseous stream. Second, only a small amount of
THF is needed together with water and therefore severe
corrosion problem can be avoided. Third, the aqueous solution
containing THF after dissociation can be easily recycled
to the hydrator.
Gas hydrates are becoming an attractive way of storing and transporting large quantities of natural gas, although there has been little effort to understand the preferential occupation of heavy hydrocarbon molecules in hydrate cages. In this work, we present the formation kinetics of mixed hydrate based on a gas uptake measurement during hydrate formation, and how the compositions of the hydrate phase are varied under corresponding formation conditions. We also examine the effect of silica gel pores on the physical properties of mixed hydrate, including thermodynamic equilibrium, formation kinetics, and hydrate compositions. It is expected that the enclathration of ethane and propane is faster than that of methane early stage hydrate formation, and later methane becomes the dominant component to be enclathrated due to depletion of heavy hydrocarbons in the vapor phase. The composition of the hydrate phase seems to be affected by the consumed amount of natural gas, which results in a variation of heating value of retrieved gas from mixed hydrates as a function of formation temperature. 13C NMR experiments were used to measure the distribution of hydrocarbon molecules over the cages of hydrate structure when it forms either from bulk water or water in silica gel pores. We confirm that 70% of large cages of mixed hydrate are occupied by methane molecules when it forms from bulk water; however, only 19% of large cages of mixed hydrate are occupied by methane molecules when it forms from water in silica gel pores. This result indicates that the fractionation of the hydrate phase with heavy hydrocarbon molecules is enhanced in silica gel pores. In addition when heavy hydrocarbon molecules are depleted in the vapor phase during the formation of mixed hydrate, structure I methane hydrate forms instead of structure II mixed hydrate and both structures coexist together, which is also confirmed by 13C NMR spectroscopic analysis.
Pyrrolidinium cation-based ionic liquids were synthesized, and their inhibition effects on methane hydrate formation were investigated. It was found that the ionic liquids shifted the hydrate equilibrium line to a lower temperature at a specific pressure, while simultaneously delaying gas hydrate formation.
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