  Let $\Lambda$ be a connected closed region with smooth boundary contained in
the $d$-dimensional continuous torus $\bb T^d$. In the discrete torus $N^{-1}
\bb T^d_N$, we consider a nearest neighbor symmetric exclusion process where
occupancies of neighboring sites are exchanged at rates depending on $\Lambda$
in the following way: if both sites are in $\Lambda$ or $\Lambda^\complement$,
the exchange rate is one; If one site is in $\Lambda$ and the other one is in
$\Lambda^\complement$ and the direction of the bond connecting the sites is
$e_j$, then the exchange rate is defined as $N^{-1}$ times the absolute value
of the inner product between $e_j$ and the normal exterior vector to
$\p\Lambda$. We show that this exclusion type process has a non-trivial
hydrodynamical behavior under diffusive scaling and, in the continuum limit,
particles are not blocked or reflected by $\partial\Lambda$. Thus the model
represents a system of particles under hard core interaction in the presence of
a permeable membrane which slows down the passage of particles between two
complementar regions.
