Understanding heat- and surface effects for zeolite membranes

Separations using zeolite membranes often require much less use of energy than conventional thermal separations like distillation. Therefore, ultrathin zeolite membranes having a large selectivity and high mass flux are currently generating a great deal of attention.

For understanding and controlling the separation selectivity and mass fluxes, detailed information on the molecular scale is needed. Surface- and heat effects are crucial for understanding membrane selectivity for the following reasons: (1) Large heat effects are associated with adsorption/desorption of molecules at the entrance/exit of the membrane. It is well known that adsorption and diffusion of hydrocarbon mixtures are strongly temperature dependent. Therefore, one cannot assume isothermal conditions at the surfaces. (2) For ultrathin membranes, the surface resistivity to mass transfer is not necessarily small compared to the intrazeolite resistivity, especially for polar compounds. (3) Assuming isothermal conditions at the boundaries leads to an incorrectly calculated heat flux and therefore an incorrectly calculated entropy production rate. Optimizing the entropy production rate is absolutely crucial for optimizing separation units.

We are using theory and molecular simulation to obtain a fundamental understanding on the transfer of heat and mass across the interface, and how the relevant coefficients vary with temperature and surface excess concentration. As two-component systems have not been studied in this manner before, significant method development and verification is required. Ultimately, this should lead to a better understanding and design of transport in zeolite membranes, as well as to factual information on transport coefficients; their magnitude and dependence on the surface intensive variables.