Distributed Modular Green Ammonia Synthesis with Absorption Separation: Understanding Rate Limitations, Insights Toward Improvement

We have been investigating a design that may better enable distributed, small-scale, modularized ammonia synthesis - particularly green ammonia made with hydrogen from sustainable wind energy. Ammonia can be separated from unreacted hydrogen and nitrogen by absorption in magnesium chloride; we have found ways to ensure the uptake/regeneration cycle is reproducible over many cycles. While we have generally found that the absorption (uptake) can be quite rapid, but the desorption (regeneration) can be rate limiting for production.

In this presentation we investigate the desorption (regeneration) step, showing that the process appears to involve two steps: (i) reactive decomposition of the ammine salt to release ammonia, and (ii) diffusion of the released ammonia through a shell of salt depleted of ammonia. The experiments and analysis, performed with lab-scale colums (grams of absorbent), can be used to design cycling performance for pilot-scale columns (kilograms of absorbent) used in a prototype module for ammonia production on the order of kilograms per day. The insights also suggest improved absorbent materials that might avert diffusion rate limitations during regeneration.