Rates of Ammonia Absorption and Release in Calcium Chloride

Abstract

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. 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.

Modeling and Optimal Design of Absorbent Enhanced Ammonia Synthesis

“…Efforts to increase the working capacity of these absorbents or to discover new ammonia absorbents with higher capacities will be beneficial in reducing process cost by reducing the size of absorbent beds and subsequently the pressure drop through the bed. Furthermore, the diameter of the currently-used absorbent pellets is on the order of 200 µm [23], an order of magnitude smaller than what is conventionally used in industrial packed beds. As a result, many tubes of absorbent are required, especially at the larger scales under investigation, to prevent prohibitively high pressure drop.…”

“…As with the reactor, the pressure drop through the bed is given by the Ergun equation (Equation (4)), and the particle Reynolds number is again in the transitional region. The bed and absorbent void fractions are determined from the experimental setup in [23]. 2 The absorbent heat capacity is assumed to be constant and is from [24].…”

“…The absorbent used in this system is selective to ammonia, and thus, the rates of absorption (and desorption) for hydrogen and nitrogen are zero. The rates of ammonia absorption and desorption were determined using data in [23]. The rate of ammonia absorption is given by:…”

Exceeding Single‐Pass Equilibrium with Integrated Absorption Separation for Ammonia Synthesis Using Renewable Energy—Redefining the Haber‐Bosch Loop

“…[ 5,29–31 ] CaCl 2 does not absorb ammonia at reasonable ammonia partial pressures above 300 °C and therefore is not suitable for a combined catalyst‐absorbent process. [ 32 ] While MgCl 2 is known to retain ammonia up to almost 400 °C, its implementation is severely hindered by its decomposition which releases hydrogen chloride. MgCl 2 absorbs water from the air (like other absorbents) and shows a strong preference to decompose to magnesium oxide and hydrogen chloride when heated above 300 °C rather than releasing water.…”

Rapid pressure swing adsorption for small scale ammonia separation: A proof‐of‐concept

“…[ 22,23 ] A major feature of the absorption‐based separation is that the absorber column, which is packed with metal halide absorbents, can separate ammonia more completely at significantly lower ammonia concentrations. [ 9,24 ] Rapid separation achieved at fast recycle rates in the reaction‐absorption process can compensate for the reduction in the reaction rates due to lower pressure processing. Unlike adsorption, which is a surface‐based phenomenon, absorption relies on gas–solid reactions followed by bulk diffusion within the solid absorbents.…”