Modeling Dry Ice Formation in Exhaust Gas Streams Using a Simplified Phase Equilibrium Approach for Multiphase Flow Coupling

CO₂ sublimation in supersonic separators requires accurate thermodynamic models to optimize separator parameters, including temperature, pressure, and component concentrations. This study examines the formation of dry ice (solid CO₂) as a method for capturing carbon dioxide from exhaust gas streams. The methodology uses fluid phase modeling with the Peng-Robinson equation of state. It incorporates empirical correlations from the literature to find the molar volume and sublimation pressure of the solid phase. The triple point criteria define the stable fluid phase for equilibrium, which is solved iteratively to determine CO₂ concentration in the gas phase. The solid mass flow rate is then calculated using a global mass balance. These models are integrated into an algorithm to calculate the sublimation point and describe solid-vapor equilibrium.

The algorithm's accuracy was validated by comparing pressure-temperature (P-T) profiles for various CO₂ compositions (3% to 100%) in a gas mixture, which demonstrated a strong agreement between the phase envelopes produced by Aspen HYSYS and the algorithm. Furthermore, the consistency is supported by the close match between the calculated vapor-phase CO₂ compositions at solid-vapor equilibrium and experimental data (Sonntag et al., 1962) across the temperature range of 140-190 K and pressures of 0-12 MPa. The results showed good agreement, confirming the model's robustness and reliability in calculating the solid-vapor equilibrium. The average relative error is 6.43%. The key claim is that this simulator-independent tool provides the computational autonomy and flexibility required to optimize the operational conditions of cryogenic carbon capture systems, thereby enhancing the efficiency of the algorithm for incorporation into Computational Fluid Dynamics (CFD) simulations at low computational cost.