Recovery of CO2 from Flue Gas Using Gas Hydrate: Thermodynamic Verification through Phase Equilibrium Measurements

Abstract

The main purpose of this study was to develop a new hydrate-based gas separation (HBGS) process especially for recovering CO2 from flue gas. Temperature and pressure conditions for hydrate formation have been closely examined at the various CO2 concentrations of flue gases. Tetrahydrofuran (THF) chosen as a hydrate promoter can also participate in forming hydrates and produces a mixed hydrate together with CO2. The hydrate stability region was greatly expanded by using THF for lowering the equilibrium formation pressure. To confirm thermodynamic validity of the HBGS process, the three-phase equilibria of hydrate, liquid, and vapor were measured for the systems comprising CO2, N2 and water with or without THF in the temperature range of 272−295 K. In addition, two phase equilibria of hydrate and vapor were experimentally investigated for the same systems at several temperatures. Through close examination of the overall experimental results, it was firmly verified that the HBGS process makes it possible to recover more than 99 mol % of CO2 from the flue gas. The key unit operations of the HBGS lie in hydrate formation and subsequent dissociation similarly to gas absorption and desorption using the sterically hindered amines. The HBGS provides several advantages over the conventional ones. First, the operational temperature is moderate in the range of 273−283 K, and continuous operation allows this process to treat a large amount of gaseous stream. Second, only a small amount of THF is needed together with water and therefore severe corrosion problem can be avoided. Third, the aqueous solution containing THF after dissociation can be easily recycled to the hydrator.