GAS PURIFICATION USING A STATE-OF-THE-ART-SOLID-VAPOR-SEPARATION-UNIT: MODELING AND SIMULATION

Abstract

In spite of the increasing levels of greenhouse gases in the atmosphere and their sever impact on the environment, the demand for natural/bio gas is expected to increase significantly in the coming decades. To cover this demand, the global energy industry is continuously exploiting sour gas reserves located around the world. Nonetheless, sour gas has to be sweetened before the practical utilization of natural or biogas. On the other hand, the combustion of fossil fuels produces flue gases that contain huge amounts of carbon dioxide (CO2) every year. CO2 is a major greenhouse gas, which has negative impact on the environmental systems. Therefore, it is important to reduce or eliminate CO2 from flue gases before being released to the environment. On the other side, radioactive isotope byproducts (e.g., krypton and xenon) are found in off-gas streams released from nuclear power plants with light water reactors or from fuel reprocessing plants, and have to be separated from the off-gas stream. To our knowledge, so far there are no known processes to separate such noble gases, so this work will be the first to address this issue. The cryogenic separation technologies have emerged as a new approach to separate carbon dioxide (CO2) and hydrogen sulfide (H2S) from natural gas, and to capture CO2 gas from flue gas streams. To design cryogenic separation equipment, vapor-liquid equilibrium (VLE), solid-liquid equilibrium (SLE), solid-vapor equilibrium (SVE), and solid-liquid-vapor equilibrium (SLVE) data for the corresponding binary and ternary systems composing the corresponding gas mixtures. In this study, we successfully develop an empirical correlation model based on the Peng-Robinson equation of state (PR EoS), with fugacity expressions, that is able to describe the SLVE behaviors for the ternary systems of CH4-CO2-H2S (resembling a sample natural gas mixture), N2-kr-Xe (resembling noble gases in nitrogen), and N2-O2-CO2 (resembling flue gas mixtures) over wide ranges of pressures and temperatures. Additionally, and based on this model, an equilibrium stage separation unit was modeled and used to construct phase diagrams of the solid-fluid regions for the above-mentioned systems. The results showed that separating the unwanted gases in the solid-vapor equilibrium (SVE) region by selective freezing (solidification) is efficient and results in high recovery rates, the recovery of acid gases (such as CO2) could exceed 99%. Based on that, and by implementing the model developed, a state-of-the art solid-vapor (SV) separation unit was modeled using the Aspen Custom Modeler (ACM) software. The SV unit was then simulated by importing the ACM code into an Aspen Plus simulator; and its performance was studied and analyzed. The SV separation unit offers some key advantages over the traditional technologies (such as amine scrubbing units);including lower energy requirements, less capital costs, lower maintenance and operation expenses and the avoidance of contaminating the gases with other components (such as solvents). In natural gas sweetening, the developed SV unit consumes only ~27% of the energy required by the amine sweetening unit; while for CO2 capture from flue gases, it saves about half of the energy needed by traditional units. For separating noble gases from nitrogen, the SV unit also achieved high recoveries, especially for xenon gas; however the operating temperature would be too low, thus requiring high energy for cooling.