Better Absorbents for Ammonia Separation

Abstract

Making ammonia from renewable wind energy at a competitive price may be possible if the conventional ammonia condenser is replaced with an ammonia absorber. Such a process change requires an ammonia selective absorbent. Supported metal halide sorbents for this separation display outstanding dynamic capacity close to their equilibrium thermodynamic limits. Alkaline earth chlorides and bromides supported on silica and zeolite Y are the most promising. MgCl2 and CaBr2 at 40% loading on silica show capacities of 60−70 mgNH3/gsorbent at 150 °C and 4 bar. Overall, cations with smaller atomic numbers show more affinity to ammonia; bromides hold ammonia more strongly than chlorides. Different solvents and metal halide mixtures do not show significant changes in the absorption capacity. These absorbents can be incorporated into ammonia reaction-absorption syntheses to achieve faster production rates.

Modeling and Optimal Design of Absorbent Enhanced Ammonia Synthesis

“…The key novelty in the process is the use of absorption rather than condensation to remove the ammonia from unreacted hydrogen and nitrogen, so naturally, the absorbent is the part of the process to which the most significant improvements can be made. In this work, absorbent capacity was estimated to be a ratio of 1 mol NH 3 to 1 mol MgCl 2 (5.6 kg of salt are needed to absorb 1 kg NH 3 ) based on the work in [12]. Coupled with the fact that the salt only makes up 40% of the absorbent mixture, every absorbed kg of NH 3 requires 14 kg of total absorbent.…”

“…These gases are compressed to the process operating pressure, which is between 15 and 30 bar (one order of magnitude lower than industrial conditions in the Haber-Bosch process), before being sent to a heat exchanger, which uses the reactor effluent to heat the feed gas to reaction temperature, which is around 400 • C. The nitrogen and hydrogen then react over a fixed bed of wustite-based iron catalyst, which is the industrial standard, to form ammonia. The reactor effluent is partially cooled using the previously-mentioned heat exchanger and further cooled with cooling water to the absorption temperature, which is in the range of 100-200 • C. The absorber is a fixed bed of magnesium chloride supported on silica [12], which serves to remove ammonia selectively through its absorption into the solid. The absorber effluent, which is primarily nitrogen and hydrogen with some unabsorbed ammonia, is then cooled to ensure safe operation of the recycle compressor, for its discharge temperature cannot exceed 150 • C. These gases are mixed with the fresh feed.…”

“…However, significantly reducing the pressure in the Haber-Bosch process is difficult because doing so reduces reactor single pass conversion and also requires even lower temperatures for condensation of ammonia from unreacted hydrogen and nitrogen, thus increasing refrigeration demand. An alternative approach is absorbent-enhanced ammonia synthesis in which a bed of supported alkali metal salt replaces the conventional condenser [11,12]. This absorbent allows for more complete ammonia separation as compared to condensation, and subsequently, high ammonia production rates can be achieved while operating at lower pressures [13].…”

Plasma-driven catalysis: green ammonia synthesis with intermittent electricity

“…101,102 These solid sorbents allow for complete ammonia removal from the reactor effluent under milder pressures of 10-30 bar and at temperatures close to the temperature of the ammonia synthesis reactor, which is coined the absorbent-enhanced Haber-Bosch process. 68,103,104 This may allow for operating the hydrogen Table 2 Comparison of alternative ammonia synthesis methods. The reported energy requirement refers to the best available technology (BAT) or the best reported value in literature.…”

“…90 Furthermore, an ammonia concentration of about 1.0 mol% (10 000 ppm) is required to minimize the energy cost of separation and recycling in case of an atmospheric synthesis loop, 95 as an ammonia partial pressure of 0.01 bar is required for effective ammonia removal in solid sorbents. 103,[273][274][275] Therefore, low pressure plasma reactors such as MW and RF plasmas require a near complete conversion at an energy yield of 100-200 g-NH 3 kW h −1 , because separation of ammonia is not feasible. The highest energy yield reported so far is Original references: DBD (AC), 144,[146][147][148][151][152][153]159,[164][165][166]169,276,277 DBD ( pulse), 158,165 Glow Discharge, 174 MW 186,278,279 and RF.…”