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 Nm3 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% H2, 25% N2, 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 H2, N2, 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®. The results, simulations and experimental, indicate that after H2 the first impurity to break thought the column is Ar, followed by N2 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 H2 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 H2, with a recovery of 75.3% and 55.5%, respectively, where the impurities were mostly Ar and N2. 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 € kg H2−1, depending on the dispensing pressure of 350 or 700 bar, respectively. These assessments offer a cost-effective solution to produce high-purity H2 as low cost transportation, allowing hydrogen penetration into the mass markets.