Phase Equilibria with Solid Carbon Dioxide for Cryogenic Capture Applications

Carbon Capture Utilization and Storage (CCUS) can play an important role in the energy transition by enabling the use of available resources while significantly reducing emissions. In recent years, this has boosted an intense research on carbon dioxide (CO2) capture technologies so that novel technologies have been developed for this purpose as alternatives to the conventional process of chemical absorption into alkanolamine aqueous solutions. Some of these novel technologies are operated at low/cryogenic temperatures and involve the formation of solid carbon dioxide (CO2(s)). This type of processes offers the advantage of separating pure CO2, which is readily available for further utilization. For a correct design of these processes, it is fundamental to be able to model phase equilibria in the presence of CO2(s). In this work, a proper model [1] is adopted to deal with this. It allows solving an isobaric-isothermal flash problem coupled to a phase stability analysis, which does not require a-priori knowledge of the number and type of phases existing at equilibrium. The reliability of the model is first assessed by comparison of its predictions with solid-vapor equilibrium data available in the literature [2,3] for the binary CO2-N2 system, which can be considered representative of a flue gas mixture as an initial approximation [4]. Then, it will be used to determine the recovery extent that can be obtained by a single-stage separation, depending on the operating conditions and on the type of flue gas CO2 has to be separated from. Indeed, the CO2 content in flue gases from natural gas- and coal-fired power plants is typically in the range 3-15 mol%. The lower limit also applies to the flue gas emitted by LNG-fuelled ships, which may be interested by on-board capture processes for reaching the targets set by the International Maritime Organization (IMO) to significantly curb greenhouse gas emissions from international shipping [5]. The final aim of this work is to provide correlations for determining CO2 recovery in the solid phase as a function of the operating conditions and of the CO2 content in the flue gas, which can be used for feasibility studies on cryogenic carbon capture technologies. References [1] De Guido, G., Spatolisano, E., 2021. Simultaneous Multiphase Flash and Stability Analysis Calculations including Solid CO2 for CO2-CH4, CO2-CH4-N2, and CO2-CH4-N2-O2 Mixtures. Journal of Chemical & Engineering Data 66(11), 4132-4147. [2] Sonntag, R.E., Van Wylen, G.J., 1962. The Solid-Vapor Equilibrium of Carbon Dioxide-Nitrogen. In Advances in Cryogenic Engineering, 99-105. [3] Smith, G.E., Sonntag, R.E., Van Wylen, G.J., 1964. Solid-vapor equilibrium of the carbon dioxide-nitrogen system at pressures to 200 atmospheres. In Advances in Cryogenic Engineering, 197-206. [4] De Guido, G., Pellegrini, L.A., 2022. Calculation of solid-vapor equilibria for cryogenic carbon capture. Computers & Chemical Engineering 156, 107569.