Layered two- and four-bed PSA processes for H₂ recovery from coal gas

Huge amounts of global warming gas emissions have prompted interest in the recovery of H₂ from off-gases in the iron and steel industries. Pressure swing adsorption (PSA) processes with layered beds packed with zeolite 5A and activated carbon were applied for H₂ recovery from coal gas with relatively low H₂ concentrations (H₂/CO₂/CH₄/CO/N₂; 38/50/1/1/10 vol.%). Breakthrough curves in the layered bed showed behavior results between the zeolite 5A bed and the activated carbon bed. The bed with the higher zeolite ratio produced H₂ of higher purity in the PSA operation, but recovery loss became more significant with its increasing ratio. The variation of purity and recovery by operating variables were more significant in the two-bed PSA process than they were in the four-bed PSA process. The purity in the two-bed PSA varied asymptotically according to P/F ratio in the range of 0.1-0.3, while purity variation in the four-bed PSA process was almost linear. The zeolite layer in the two-bed PSA process worked as a separator of N₂, while that in the four-bed PSA process worked as a purifier of N₂. The four-bed PSA process could produce H₂ with a purity of 96-99.5% and a recovery of 71-85% with N₂ as the major impurity. The dynamics of the breakthrough and H₂ PSA processes were studied using a non-isothermal dynamic model.