Ammonia absorption at haber process conditions



The kinetics of ammonia absorption into magnesium chloride is measured as ammonia pressure change in the temperature range 170-430°C. The actual pressure minus that at equilibrium drops quickly, with a half life of less than a minute. It varies with the square-root of time, suggesting diffusion limited absorption. The diffusion coefficient of ammonia in solid magnesium chloride inferred from these data is on the order 10−11-10−13 cm2/s, considerably faster than many solid-phase diffusivities. While optical microscopy and BET surface area experiments indicate recrystallization and agglomeration of the absorbent at ammonia synthesis temperatures, the absorption rate remains high. The dependence of absorption rate on temperature, particle size and the presence of a silica support is also investigated. The results suggest both improved ammonia separation and ways to develop high-conversion, small-scale, multifunctional ammonia synthesis reactors. © 2012 American Institute of Chemical Engineers AIChE J, 2012