A Grand Canonical Monte Carlo Study of the N₂, CO, and Mixed N₂–CO Clathrate Hydrates

Abstract

In this paper we report Grand Canonical Monte Carlo simulations to characterize the competitive trapping of CO and N 2 molecules into clathrates, for various gas compositions in the temperature range from 50 to 150 K. The simulations evidence a preferential trapping of CO with respect to N 2 . This leads to the formation of clathrates that are preferentially filled with CO at equilibrium. This is irrespective of the composition of the gas phase, the fugacity and the temperature. Moreover, the results of the simulations show that the small cages of the clathrate structure are always filled in first place, and this is independent of either the guest structure or the temperature. This issue has been associated to the rather significant differences in the calculated heats of encapsulation (∼2-3 kJ/mol) between the small and the larges cages. In addition, calculations with the simplified ideal adsorbed solution theory (IAST) are developed for allowing a comparison with the results arising from the GCMC simulations. Interestingly, this shows that the occupancy isotherms of the mixed N 2 -CO clathrates can be perfectly represented by knowing the occupancy isotherms of the corresponding singleguest clathrates. This suggests that experiments performed with the single guest CO and N 2 clathrates might be sufficient to get information concerning the corresponding mixed clathrates by using the IAST approach.

Cage Occupancies in Nitrogen Clathrate Hydrates from Monte Carlo Simulations

“…2) by setting γ to 12.2, a value obtained by fitting the N O potential of Ref. 22 (blue curve) to a Buckingham interaction. This provides the parameters listed in the first column of Table 2.…”

“…The remaining columns show parameters used in previous studies: Ref. 22 for the case of potential 1 with TIP4P-Ew water and Refs. [19][20][21]30 for the case of potential 2. calculated from the ab-initio potential energy surface of Tulegenov et al 33 (circles) and its fit to an Exp-6 interaction (black line).…”

“…The potential set 1, when combined with TIP4P-Ew water, coincides with the interaction potentials used in Ref. 22…”

“…As explained in the previous section, a tendency for the small cages to be more occupied than the large cages, like in the experimental dataset for P ≤ 100 bar, is expected at low temperatures. From the simulations of Ref., 22 the transition from C L (T ) > C S (T ) to C L (T ) < C S (T ) is expected to occur at around 80 K. Even with such an "inversion" of the Langmuir constants, the cages are still expected to be fully occupied at the considered pressures, which are much larger than the dissociation pressure (this can be confirmed by extrapolating to higher pressures the simulation data at 50 K, 100 K and 150 K of Ref. 22 ).…”

“…1 Patt et al performed GCMC simulations of N 2 , CO and mixed N 2 CO clathrates but only at low temperatures (50, 100 and 150 K) and they did not compare quantitatively with experimental data. 22 Their calculations suggest that experiments performed on the single-guest N 2 and CO clathrates might be sufficient to get information on the corresponding mixed clathrates.…”