Permeation of gas mixtures through zeolite membranes

Permeation of single gases and mixtures containing CO2, H2 and CO was investigated through NaY and DD3R zeolite membranes. Mass transport was modelled considering the competition between surface and gas translation diffusion.1 In particular, it was demonstrated the importance of gas translation also at a low temperature, in the region where surface diffusion is generally the dominant mechanism. Specifically, the permeation of weakly adsorbed species (i.e., H2) through NaY membrane was well predicted in the whole temperature range experimentally investigated in the literature.2 In fact, this model is able to describe the increase of H2 permeance at low temperatures owing to both surface and gas translation diffusion. Furthermore, a maximum in permeance was predicted at about 450 K, whereas gas translation becomes more important than surface diffusion above 520 K. A previous model, 3 which considered the competition between surface and Knudsen diffusion, showed a discrepancy with the experimental values at low temperatures. In fact, Knudsen diffusion instead of gas translation provides an error in H2 permeance of about 30% at 308 K. On the contrary, the strongly adsorbed species are not dependent on gas translation or Knudsen diffusion, since the surface diffusion is the dominant mechanism in the whole temperature range investigated. The estimated CO2/H2 mixture selectivity value at 303 K by this model in NaY membrane coincides with the experimental value of Kusakabe et al. 308 K.4 In fact, the same selectivity of 28 was obtained. On the other hand, a lower value of about 20 was predicted using Knudsen instead of gas translation. Permeation of light gases through a DD3R zeolite membrane was also evaluated considering the competition between surface and gas translation diffusion. The experimental permeances of H2, CO and CO2 as a function of temperature measured by Kanezashi et al.5 were well described in the whole temperature range considered. A presence of a minimum was predicted for CO2, CO and H2 at 690, 480 and 440 K, respectively. The minimum of H2 was also experimentally observed.5 The CO2 permeation takes place by surface diffusion up to a relatively high temperature. The minimum of CO2 permeance was found between the experimental values at 673 and 773 K (at ca. 690 K). The CO permeation occurs only by surface diffusion up to 400 K, whereas gas translation becomes significant above 450 K. The H2 permeation at 298 K is only owing to surface diffusion, which drastically tends to decrease with increasing temperature. On the other hand, gas translation becomes important with increasing temperature, being higher than surface diffusion above 410 K. The correct description of single gas behavior allowed a prediction of the separation performance through DD3R membranes also for mixtures. In particular, syngas permeation through a DD3R membrane was simulated in a wide range of temperature (273 -813 K) and feed pressure (200 - 1000 kPa). This membrane showed CO2/CO and CO2/H2 selectivity values of about 63 and 56 respectively at 273 K, which decrease to 36 and 18 at 303 K. A reverse H2/CO2 selectivity was predicted above 450 K. An increment of feed pressure did not significantly affect CO2/H2 selectivity, whereas slightly reduced the CO2/CO selectivity. Therefore, NaY and DD3R are good candidates for separation of different gas mixtures. Gas translation competing with surface diffusion successfully predicts the experimental permeation through zeolite membranes for both mixture and single gas at different operating conditions.