The diffusion of ethane and ethylene in large crystals of DDR clathrisil (pure silica analogue of ZSM-58) was studied (figure 22) by using the zero-length column technique. The behaviors of ethane and ethylene were very similar with respect to both kinetics and equilibria. Measurements were made of two different DDR samples, with both He and CO2 as the carrier gas. In contrast to methane, neither the equilibrium nor the diffusivity of ethane (or ethylene) was significantly affected by the presence of CO2; suggesting non-competitive adsorption. Although the diffusivity of the C2 hydrocarbons was about an order of magnitude smaller than that of methane, the activation energies were similar or even smaller. In contrast to the behavior of methane, the data furnished clear evidence of a small but significant surface resistance (in addition to the intracrystalline diffusional resistance).

Diffusion of C2H6 and C2H4 in DDR Zeolite. A.Vidoni, D.M.Ruthven: Industrial and Engineering Chemistry Research, 2012, 51[3], 1383–90

Figure 22

Diffusivity of Ethane in ZSM-58 Analogue DDR

(a: mean crystal radius 20μ, b: mean crystal radius 10μ)