Palladium (Pd) membranes are characterized by their high permselectivity to hydrogen and easy operation, and are promising devises for separating hydrogen from hydrogen-rich gases. The membranes are normally operated with atmospheric pressure at the permeate side. Instead of this common operation, hydrogen permeation through a Pd membrane under vacuum operation at the permeate side is investigated and compared with that under normal operation. In this study, two membrane operating temperatures (320 and 380 degrees C), four H-2 partial pressure differences (2, 3, 4, and 5 atm) across the membrane, and four feed gases are considered. The results suggest that the vacuum operation can efficiently intensify the H-2 permeation rate. The improvement in H-2 permeation rate due to the vacuum operation can be increased up to 136%. The positive effect of the vacuum operation is especially pronounced when the gas mixtures are used as the feed gases, stemming from the effective attenuation of the concentration polarization. An increase in membrane temperature raises the H-2 permeation rate, but its influence in enhancing the H-2 permeation rate with the vacuum operation is not as significant as that without the vacuum one. It is found that the retardation effect of impurities on the mass transfer is always ranked as CO > CO2 > N-2, regardless of with/without vacuum operation. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.