Noura Dawass

6 122 S OLUBILITY OF G ASES IN M ONOETHYLENE G LYCOL 6.3. R ESULTS AND DISCUSSION 6.3.1. S OLUBILITY OF CO 2 IN MONOETHYLENE GLYCOL Densities of pure MEG were computed using MC simulations in the NPT en- semble at P = 1 bar, and at T = 333.15 K and T = 353.15 K. In Table 6.2, densi- ties (reported in units of kg/m 3 ) from simulations are compared to experimental values from the work of Skylogianni et al. [184] . Table 6.2 shows that when us- ing the TraPPE-UA force field, simulations underpredict densities of MEG. The differences between experiments and simulations are around 5%. Simulating a solvent with an underestimated density can result in higher absorption capacity. Deviations between experiments and simulations will be discussed in detail later in this section. As a fractional MEG molecule is present in the simulation, we can calculate the excess chemical potential of MEG µ ex from the probability distribution of its λ parameter [60, 196] . The chemical potential of MEG in the liquid phase equals [196] : µ L = µ o + RT ln ρ L ρ o + µ ex (6.2) where ρ L is the number density of MEG and µ o is the reference chemical poten- tial, which only depends on the temperature. Eq. (6.2) also applies to MEG in the gas phase. At equilibrium, the chemical potentials in the liquid phase and gas phase are equal. If we assume an ideal gas phase, then µ ex in the gas phase equals zero and ρ L = P sat / k B T . From this, the saturated vapor pressure P sat of MEG can be estimated by P sat = k B T ρ L exp · µ ex k B T ¸ (6.3) For a detailed derivation of Eq. (6.3) , see the Supporting Information of Ref. [171] . In Table 6.2, we report the excess chemical potential, vapor densities and saturated vapor pressures of pure MEG. The vapor pressures computed from MC simulations are compared to experimental values obtained from NIST database [222] . Table 6.2 shows that both methods are in good agreement. The pressures reported in Table 6.2 can be considered very small, which validates the assumption made by the experimental method regarding the non–volatility of MEG. Figur e 6.3 shows absorption isotherms of CO 2 inMEG, from experiments and MC simulations in the osmotic ensemble, at temperatures T = 333.15 K, T = 353.15 K, and T = 373.15 K. In Figure 6.2, a typical MC simulation snapshot of MEG and CO 2 molecules is shown. Figure 6.2 shows that CO 2 molecules are dis- persed in MEG and not clustered. In Figure 6.3, the ratio of the number of moles