Optimizing Cyclic PSA/TSA Conditions for Ammonia Separation with Supported Metal Halides

The recent push towards reduction of carbon dioxide emissions has been the main reason for finding alternatives to the conventional Haber-Bosch ammonia production process. Although well-optimized through a century of research and development, the high operating pressure of the Haber-Bosch process is economically demanding. Alternative pathways to produce green ammonia introduce renewable energy (mostly wind-generated) as a base for ammonia production with the downside being double the cost. This shortcoming is mitigated by operating the process at lower pressure by replacing the condenser with an absorber containing metal halides capable of better ammonia separation – using the reaction-absorption process.^1–4 The application of absorption for ammonia separation is well studied; yet, there is no information about the rates of ammonia release from the metal halide absorbents.^5,6

In this presentation, we offer one of the initial efforts to study ammonia release from different metal halide absorbents. The target is to better understand the ammonia release rates and absorbent working capacity under the cyclic pressure and temperature swing absorption conditions (PSA/TSA). A lab-scale absorber packed with supported and unsupported materials is used to observe ammonia breakthrough and release times for each cycle. Ammonia release at specific temperatures and pressures is recorded in carefully controlled experiments. Absorption tests were conducted at 40 psi with temperatures in the range of 25-400 °C. Results show a viable path to achieve reproducible breakthrough curves while achieving ammonia concentrations above 90%. The effect of the sweep gas during the desorption is also studied with the purpose of finding the optimal values for sweep time and volumetric flow. These findings will guide us to further optimize the small reaction-absorption ammonia production process and explore the strategies for further scale-up.

References:

(1) Malmali, M.; Wei, Y.; McCormick, A.; Cussler, E. L. Ammonia Synthesis at Reduced Pressure via Reactive Separation. Ind. Eng. Chem. Res. 2016, 55 (33), 8922–8932.

(2) Malmali, M.; Reese, M.; McCormick, A. V.; Cussler, E. L. Converting Wind Energy to Ammonia at Lower Pressure. ACS Sustain. Chem. Eng. 2018, 6 (1), 827–834.

(3) Reese, M.; Marquart, C.; Malmali, M.; Wagner, K.; Buchanan, E.; McCormick, A.; Cussler, E. L. Performance of a Small-Scale Haber Process. Ind. Eng. Chem. Res. 2016, 55 (13), 3742–3750.

(4) Cussler, E.; McCormick, A.; Reese, M.; Malmali, M. Ammonia Synthesis at Low Pressure. J. Vis. Exp. 2017, No. 126, e55691–e55691.

(5) Wagner, K.; Malmali, M.; Smith, C.; McCormick, A.; Cussler, E. L.; Zhu, M.; Seaton, N. C. A. Column Absorption for Reproducible Cyclic Separation in Small Scale Ammonia Synthesis. AIChE J. 2017, 63 (7), 3058–3068.

(6) Malmali, M.; Le, G.; Hendrickson, J.; Prince, J.; McCormick, A. V.; Cussler, E. L. Better Absorbents for Ammonia Separation. ACS Sustain. Chem. Eng. 2018, 6 (5).