(333d) Replacing Phase Changing Separations: Revising Ammonia Manufacturing

With more than 150 million metric tons annual production, ammonia is the second most produced commodity chemical, and its production is the largest energy-consumer in the chemical industry. Ammonia production is complicated, and it is carried out at high temperature (>400 °C) and high pressure (150-300 bar.) High temperature is required to accelerate splitting the nitrogen triple bonds, while high pressure is required to ensure reasonable single-pass conversions. Ammonia separation relies on phase-changing separations, which requires high pressure processing for a cost-effective separation. High pressure processing imposes substantial operating (e.g., compression) and capital (advanced costly alloys, thick reactor casing, etc.) expenses. The gas mixture that leaves the reactor at ~450 °C should be cooled down to -20 °C in order to separate ammonia, necessitating significant heat exchanging. These are major issues that complicates ammonia manufacturing, which directly roots in the nature of phase-changing separation.

In this presentation I will discuss an innovative process for ammonia manufacturing: high temperature reaction-absorption process. In this process, we replaced the condenser with absorber column, packed with cheap abundant metal halides. These materials have extraordinary capacity for ammonia (e.g. 100 g of MgCl2 can absorb up to six moles of ammonia), even at temperatures as high as 300 °C. This separation unit allows lower pressure operation. Unlike condensation which is a well-mixed unit operation, the proposed absorber column is considered a staged unit. It allows more complete separation of ammonia at lower pressure, unless breakthrough occurs. I will present our recent findings about the mechanism of reaction-absorption process and novel absorbent materials that we have developed for ammonia separation.