PSA purification of waste hydrogen from ammonia plants to fuel cell grade

Abstract

Industrial hydrogen-rich waste streams hold promises in their upgrading to feed fuel cell stacks. As in the ammonia synthesis process, a stream of up to 180-240 Nm(3) per ton of ammonia is purged to keep the inert gases concentration below a threshold value; this stream contains large hydrogen quantities, which could be recovered. In the current work, a four-column PSA unit has been used to produce a high-purity hydrogen stream for fuel cell applications from a synthetic mixture with a molar composition of 58% H-2, 25% N-2, 15% CH4 and 2% Ar, based on ammonia purge gas. Firstly, a comparative performance of four commercially adsorbents was accomplished to obtain the adsorption isotherms of H-2, N-2, CH4, and Ar, leading to the selection of 5A zeolite adsorbent. Then, the dynamic behavior of a packed bed was studied by single and multicomponent breakthrough experiments and simulated using Aspen Adsorption (R). The results, simulations and experimental, indicate that after H-2 the first impurity to break thought the column is Ar, followed by N-2 and finally by CH4. Then, a design-of-experiments (DoE) methodology was used to select the best operating conditions of the experimental cyclic PSA unit to reach different target hydrogen product concentrations; the overall PSA performance was evaluated in terms of purity and recovery of H-2 product. According to the results, the four-column PSA unit running at 9 bar produced a stream with hydrogen concentration of 99.25% and 99.97% of H-2, with a recovery of 75.3% and 55.5%, respectively, where the impurities were mostly Ar and N-2. In addition to the technical performance, the economic assessment concluded that the cost to compress, transport and purify waste hydrogen to a concentration of 99.97% using a small-scale PSA unit from ammonia plants has been estimated in the range of 1.17-1.39 (sic) kg H-2(-1), depending on the dispensing pressure of 350 or 700 bar, respectively. These assessments offer a cost-effective solution to produce high-purity H-2 as low cost transportation, allowing hydrogen penetration into the mass markets.