Soret effect on the diffusion of CO_2 in aqueous solution under high-pressure

摘要

Diffusive dissolution processes of CO_2 into aqueous solution subjected to temperature gradients were investigated under 20 and 30 MPa in a silica capillary tube. The temperature and concentration profiles were measured simultaneously by quantitative Raman spectroscopy. Thermodynamic CO_2 saturated concentration profiles along the temperature gradients in horizontal direction at steady state were observed, which indicates that the thermodynamic contribution is dominant and kinetic contribution can be neglected. Based on the assumption of thermodynamic saturated concentration profile at steady state, Soret coefficients (the ratio of the thermal and mass diffusion coefficients, S_r) of dissolved CO_2 were calculated at P-T ranges from 0.1 to 50 MPa and from 273.15 to 623.15 K. At constant pressure, S_r shows a "V" shaped non-monotonic function of temperature with a minimum at T_0. The temperature T_0 (at which S_r reaches its minimum value) increases with increasing pressure. S_r generally decreases with increasing pressure, and could change sign from positive to negative in the temperature ranges around T_o at pressures higher than 7.5 MPa. Soret coefficients are discontinuous near the critical point of CO_2.