Achieving +95% Ammonia Purity by Optimizing the Absorption and Desorption Conditions of Supported Metal Halides

The worldwide push toward the reduction of carbon dioxide emissions has been the main motivation for finding a sustainable alternative to the conventional Haber-Bosch ammonia production process that has a significant carbon footprint. In this work, we focused on ammonia separation by replacing the condenser with an absorber column packed with metal halide solid absorbents. These salts had shown promise in selective separation of NH3 in the past, but more information on the cyclic operation and ammonia desorption conditions was needed. We used an automated apparatus equipped with an absorption column packed with either silica, supported CaCl2, or supported MgCl2 to explore the optimal absorption/desorption conditions (pressure and temperature swings). Primarily, we are reporting on the working capacity of various sorbents for cyclic ammonia separation. Additionally, we investigated the effect of sweep gas on the desorption efficiency and compared the absorbent performance among each other in terms of absorption working capacity and the purity of the ammonia product stream. We were able to achieve an NH3 stream with a purity of over 95%; in some of the tests, we achieved a coordination number as high as 2.5 molNH3/molsalt, which is the highest ever reported for a dynamic flow breakthrough test. Our experiments further prove the significant potential that these salts possess to replace phase change condensation in the conventional ammonia synthesis-not only in a greener fashion but also more efficiently with a decreased equipment size, with reduced energy input in smaller scales, and with more flexibility to follow intermittent renewables.