Ammonia Absorption at Haber Process Conditions Mark S. Huberty, Andrew L. Wagner, Alon McCormick, and Edward Cussler Dept. of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN, 55455 DOI 10.1002/aic.13744 Published online in Wiley Online Library (wileyonlinelibrary.com). 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 cm 2 /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. V V C 2012 American Institute of Chemical Engineers AIChE J, 00: 000–000, 2012 Keywords: absorption, diffusion Introduction For the last century, the chemical industry has depended on feedstocks of fossil fuels. These feedstocks can be deliv- ered centrally, from a pipeline, a tanker, or a train of coal cars. The chemical industry that has evolved under these conditions is highly centralized around large, carefully opti- mized plants. It has effectively produced large quantities of inexpensive fuels, textile fibers, and fertilizers, which have supported the development of modern civilization. Now, the limits on fossil fuel reserves may change this struc- ture. The manufacture of small amounts of highly specialized chemicals, like pharmaceuticals, will still occur in a few speci- alized plants. However, the manufacture of some fuels and fer- tilizers may take place in small dispersed plants. One good example is liquid fuels made from biomass. These are less likely to be made in large complex plants because collecting the biomass will take too much energy. Such biomass-based fuels are more likely to be made in small simple plants making prod- ucts for a local market. Thus, a new commodity chemical indus- try may depend on distributed, not centralized, manufacture. Ammonia, the key fertilizer responsible for the green rev- olution, is one compound which may be made in this distrib- uted fashion. Ammonia is made from nitrogen and hydrogen. Nitrogen can be separated from air at large scale by cryo- genic distillation and at small scale using selective hollow fiber membranes. Hydrogen can be made at large scale from the incomplete combustion of natural gas, or at small scale from the electrolysis of water. The nitrogen and hydrogen are then combined at 120 bar and 380  C to make ammonia. These high pressures and temperatures are needed to achieve fast reactions, typically taking less than a minute. However, single-pass conversion is modest, around 20%, 1 so the pro- cess requires separating the ammonia, and recycling the unreacted nitrogen and hydrogen. This separation and recycle adds complexity to the process. Such complexity can make distributed manufacture of ammonia less likely. In this article, we seek a solid absorbent for ammonia which can enhance the effective conversion and simplify am- monia synthesis. The absorbent should be able to operate at the current reaction temperature and pressure. It should oper- ate over many cycles of loading and unloading. Most impor- tantly, the absorption and desorption should be fast, about as fast as the reaction itself. In this article, we test magnesium chloride as a possible ab- sorbent for ammonia. Studies in the literature show that this compound can absorb much more ammonia than other, more conventional materials, as shown in Table 1. 2–7 The absorbent takes up ammonia in three distinct complex formation steps MgCl 2ðsÞ þ NH 3ðgÞ Ð MgðNH 3 ÞCl 2ðsÞ MgðNH 3 ÞCl 2ðsÞ þ NH 3ðgÞ Ð MgðNH 3 Þ 2 Cl 2ðsÞ MgðNH 3 Þ 2 Cl 2ðsÞ þ 4NH 3ðgÞ Ð MgðNH 3 Þ 6 Cl 2ðsÞ Absorption at lower temperatures uses all three reactions as a means of storing energy for automobiles: the ammonia is released from the magnesium chloride by warming and cata- lytically decomposed into hydrogen, which is then burned in an engine. 8 Absorption at the high ammonia synthesis reactor temperatures uses only the first reaction. However, the mecha- nism and the kinetics of this absorption at these temperatures are not known, and are explored in this article. Theory We will measure the kinetics of ammonia uptake as changes in ammonia pressure in a small cell of fixed volume Correspondence concerning this article should be addressed to E. Cussler at cussl001@umn.edu. V V C 2012 American Institute of Chemical Engineers AIChE Journal 1 2012 Vol. 00, No. 0