2.3.4. Clathrate Hydrate Process

Gas hydrate or clathrate hydrates are ice-like crystal compounds formed by water molecules and a number of other substances including CO2, N2, H2, and O2. These small gas molecules become trapped inside cavities of water molecules [72]. Concentrations of different gases in the crystals are different from their concentrations in the original gas mixture [73].

The main concept of separating CO2 is the partition of the CO2 selectively from a gas mixture between the solid hydrate crystal phase and the gaseous phase by forming a hydrate crystal. Thermodynamically, the minimum pressure to form hydrate at a temperature of 273.9 K is 5.56 MPa. The pressure of syngas after the water gas shift reaction is normally 2–7 MPa, whereas the flue gas in post combustion is almost at atmospheric pressure. Therefore, the gas stream requires compression to increase the rate of hydrate formation [74].

Different promoters have been tested to reduce the equilibrium condition to form hydrates. The most studied promoter is tetrahydrofuran (THF). Equilibrium of hydrate formation reduces with the addition of THF at any temperature. Increasing the concentration of THF causes a decrease in the hydrate formation pressure up to an optimum concentration ~1 mol% THF. It can be used in CO2 separation industrially without compressing the flue gas significantly [75]. The equilibrium pressure to form hydrate may reduce by 50% if 3.2 mol% propane is added to a CO2/O2 mixture [76].

For the fuel gas mixture in the precombustion process, the hydrate phase equilibrium condition is reduced with the addition of tetra-n-butyl ammonium bromide (TBAB). Hydrate formation condition decreases with an increase in TBAB concentration up to the stoichiometric condition, beyond which the phase equilibrium increases with by increasing TBAB concentration [77]. Park et al. [78] investigated the effect of quaternary ammonium salts on hydrate formation. They showed that 95% of CO2 can be captured from an IGCC using just one step of hydrate formation. TBAF showed a better result than TBAB but with a lower gas uptake.

Recent studies have also focused on the type of reactors. Zheng et al. [79] studied the impact of bed volume and bed reactor orientation for hydrate formation in precombustion carbon capture. Horizontal orientation performed better than vertical configuration. They also showed that low water saturation is preferable to form more hydrate.
