Absorbents For Ammonia Capture and Storage: Structure and Composition for Stability, Capacity, and Transport

Metal halides such as MgCl2 have a high capacity for ammonia absorption and can be used in place of a condenser to efficiently separate ammonia from reactor effluent in the Haber-Bosch process. Although these salts are effective at capturing and storing ammonia, they show reduced working capacity in their pure form with repeated cycling due to slowed uptake and release with particle agglomeration and heat transfer limitations. To stabilize ammonia uptake performance, MgCl2 salt crystals were supported on aluminum fibers. Incorporating aluminum fibers into pure MgCl2 was found to reduce sample charging during electron microscopy and increase bed thermal conductivity. By using a 40 wt.% MgCl2-Al absorbent mixture, ammonia storage capacity was increased 33%, and the absorbent bed thermal conductivity increased by two orders of magnitude over our previously reported absorbent (40 wt.% MgCl2-SiO2). Higher stability of the aluminum-supported MgCl2 was attributed to sorbent surface area stabilized by the aluminum fibers, which guided recrystallization events to maintain small crystals that adhered to the metal surface; thermal conductivity of the metal fibers also suppressed temperature overshoot or undershoot during ammonia uptake or release, respectively.