Improving Absorbent-Enhanced Ammonia Separation For Efficient Small-Scale Ammonia Synthesis

Synthesized ammonia exiting a reactor with hydrogen and nitrogen can be selectively absorbed by MgCl2 for renewable absorbent-based Haber–Bosch for dispersed ammonia manufacturing. Such separation can be more efficient even at elevated temperatures compared to the condensation method used in the conventional Haber–Bosch process. To determine the optimal conditions to capture and release the most ammonia per thermal cycle of the sorbent salt, the sorbent capacity was measured with varying regeneration temperature, regeneration time, and sweep rate under steady-state cycling conditions. In all cases, uptake was limited to bed breakthrough, and cyclic steady state was achieved. By using a lower temperature for MgCl2 regeneration (200 °C), the working capacities were maintained comparable to those at higher desorption temperatures (∼400 °C), even without the use of inert sweep gas. Using a sufficiently high regeneration temperature (∼200 °C) allowed for sufficiently low sweep gas so that the product ammonia can exceed 72 mol % purity in a mixture of N2 and H2. These results were achieved with a short regeneration time of 20 min or less, which is an improvement from the hour-long regeneration time previously reported. These measurements identified new operating parameters for more efficient absorber design to produce economical renewable ammonia at small scale.