  The collision dynamics of hard spheres and cylindrical pores is solved
exactly, which is the minimal model for a regularly porous membrane.
  Nonequilibrium event-driven molecular dynamics simulations are used to show
that the permeability $P$ of hard spheres of size $\sigma$ through cylinderical
pores of size $d$ follow the hindered diffusion mechanism due to size exclusion
as $P \propto (1-\sigma/d)^2$. Under this law, the separation of binary
mixtures of large and small particles exhibits a linear relationship between
$\alpha^{-1/2}$ and $P^{-1/2}$, where $\alpha$ and $P$ are the selectivity and
permeability of the smaller particle, respectively. The mean permeability
through polydisperse pores is the sum of permeabilities of individual pores,
weighted by the fraction of the single pore area over the total pore area.
