Engineering topics
Cussler, E. L.
Publications and source records attributed to Cussler, E. L..
Optimizing Ammonia Separation via Reactive Absorption for Sustainable Ammonia Synthesis
Metal halide salts such as magnesium chloride have been shown to be promising candidates for ammonia storage materials for energy storage and agriculture applications due to their ability to incorporate several moles of ammonia per mole of salt. Ammonia exiting a synthesis reactor can be separated from nitrogen and hydrogen by absorption into magnesium chloride. Such an absorption can be more complete and hotter than separation via ammonia condensation, the current standard in the Haber–Bosch process. Here, we discuss the optimal conditions for the cyclic uptake and release of ammonia from the supported magnesium chloride absorbents. An automated system was designed for measuring the nonequilibrium working capacity of the absorbent, as well as the impact of important operating conditions such as absorption and desorption temperature, pressure, and desorption time. Measurements of absorption and desorption kinetics provide insight into the mechanisms involved. The temperatures and pressures during absorption and desorption were designed to use minimal energy input to maximize the uptake and release of ammonia within a reasonable amount of time. In a laboratory-scale bed, absorption has a small unused bed length, so it is largely independent of mass transfer; it is dominated by how fast ammonia is fed to the bed. Yet, desorption is restricted both by the speed of heating the bed and by diffusion out of the absorbent. These measurements provide guidelines for ammonia separations and cycling sorbent materials on a larger scale.
Optimizing the Conditions for Ammonia Production Using Absorption
Ammonia is made from hydrogen and nitrogen over a catalyst operating at high temperature and pressure. More ammonia can be produced by changing how the ammonia synthesized is separated, i.e., by replacing the current condensation of ammonia with absorption in salts like magnesium chloride. This paper uses here the concept of resistances in series in conjunction with experiments of absorption and a well-established theory of reaction to identify conditions where the rate of ammonia synthesis can be increased. For example, in one case, if a condenser in a conventional process is simply replaced with an absorber, the increase in production rate per mass of catalyst is capped at 10%. However, if the recycle rate is simultaneously increased, the increase in production rate per gram of catalyst can exceed 1000%.
Rates of Ammonia Absorption and Release in Calcium Chloride
After synthesis, ammonia can be selectively absorbed by calcium chloride; nitrogen and hydrogen are not absorbed. The kinetics of release seem to be diffusion controlled. The kinetics of absorption are consistent with a first-order reaction after an initial period of a higher-order reaction, which may indicate a nucleation event. At 225 °C, both absorption and release show half-lives of around 10 min if the ammonia partial pressure is swung from 2 to 1 bar, which allows design of an absorber for the periodic separation of ammonia. Furthermore, when this absorber replaces the ammonia condenser in a conventional ammonia synthesis, the ammonia production at low pressure can have the same rate as the conventional process operating at higher pressure.