A model to optimize the selectivity of gas separation in membranes

We present a model of olefin/paraffin separation via nanocomposite membranes and a corresponding lattice kinetic Monte Carlo simulation. Our model is based on the solution-diffusion theory of facilitated transport through microporous membranes and the preferential binding of olefins to silver ions and nanoparticles. In our model, olefin molecules bind to the randomly distributed traps on a lattice whereas paraffin molecules do not. Selectivity for olefins over paraffins in the steady state is explained in terms of lattice diffusion and the equilibrium statistics of adsorbed gases. We show that, to first order, the maximum selectivity occurs when the rate at which an olefin molecule enters the lattice is the same as rate at which it leaves a trap site. The maximum selectivity is higher for weaker binding traps than for strong binding traps. Our model also demonstrates that the maximum selectivity increases with nanoparticle loading in the membrane and that, when the selectivity is maximized as a function of olefin binding and applied pressure, the olefin permeability is reduced from the neat membrane by a constant factor.