A multiscale study of MOFs as adsorbents in H₂ PSA purification

In this multiscale study, four robust zirconium oxide based metal-organic frameworks (MOFs) were examined using powerful molecular simulation tools as well as indispensable full-scale PSA system modeling to determine their potential for H₂ purification. Grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations were employed to evaluate the MOF working capacities, binary-mixture selectivities, and micropore transport diffusivities for each of the components of a steam methane reformer offgas (SMROG) stream: H₂, CO, CH₄, N₂, and CO₂. The small, functionalized pores of UiO-66(Zr)-Br were found to result in high N ₂ and CO selectivities and working capacities, whereas the slightly larger pore volume of UiO-66(Zr) favored higher CO₂ and CH ₄ working capacities. The collective impact of impurity uptakes and selectivities on the purification of H₂ from five-component steam methane reformer offgas mixtures was investigated through PSA column modeling. The breakthrough behavior of SMROG mixtures in columns containing MOF crystallites was determined using the simulated adsorption and diffusivity data as input. MOF breakthrough curves for single and two-layered beds were compared to those of commercial adsorbents including Zeolite 5A and Calgon PCB. Two of the MOFs, namely, UiO-66(Zr) and UiO-66(Zr)-Br, were found to have longer dimensionless breakthrough times than the commercial zeolite materials and are therefore expected to result in larger yields of high-purity H₂ per PSA cycle. UiO-66(Zr)-Br was found to be the most promising of the four MOFs, having the longest dimensionless breakthrough times in both single and two-layered bed setups.