Thermophysical property prediction of supercritical multiple mixtures fluid by molecular dynamics simulation

Abstract

Supercritical multiple mixtures (H₂O/CO₂) fluids are produced in the poly-generation system based on supercritical water gasification (SCWG). Understanding the thermophysical property of supercritical multiple mixtures fluid is rather essential to design and industrialize the poly-generation system. At present, the selection of force field models for H₂O/CO₂ mixtures are rather complicated, chaotic and inconsistent, the calculation of each parameter necessitates the use of one force field model. There is no universal force field model to calculate and conduct thermophysical property of H₂O/CO₂ at one time in a wide range of temperature within SCW. In this paper, heat capacity, viscosity and thermal conductivity of multiple mixtures in supercritical and near-critical water (575 K–1075 k) are first investigated via a novel common force field model with molecular dynamics (MD) simulations. The effects of different factors including the pressure, temperature and CO₂ mass fraction on thermophysical properties of H₂O/CO₂ at a wide range of temperature were first analyzed and conducted in detail. The calculated results indicate that the phenomenon “critical point drift” for viscosity and thermal conductivity of H₂O/CO₂ mixtures migrating toward the low temperature region occurs. The drift values of viscosity and thermal conductivity are about 10 K and 28 K. The variation of heat capacity and viscosity of H₂O/CO₂ mixtures increases with increasing pressures only in low temperatures region (700 K-–800 K), while the thermal conductivity always increases with increasing pressures. When the mass fractions of CO₂ increases from 20 % to 40 % and 60 %, the drift values for thermal conductivity and viscosity of H₂O/CO₂ mixtures decreases from 628 K to 624 K and 620 K, and from 627 K to 618 K and 610 K, respectively, and the peak heat capacity decreases from 64.9 J/mol·K to 52.4 J/mol·K and 45.1 J/mol·K. The fitting polynomial to determine conductivity, viscosity and heat capacity H₂O/CO₂ mixtures as a functional relationship with temperature were first obtained. The research data of this paper could facilitate the industrialized application of the poly-generation system.