  Using a Wigner-Seitz approximation with spherical cells, we re-analyze a
widely used single nucleus model of accreting neutron star crust. We calculate
beta disequilibrium within the crust, which is sizable, and implies that
neutron and baryon chemical potentials, $\mu_n$ and $\mu_{\rm b}$, are not
equal. We include also non-equilibrium reactions, driven by matter compression,
and proceeding in the reaction layers. The constancy of $e^{\Phi}\mu_n$, where
the spacetime metric component $g_{00}=e^{2\Phi}$, in the shells between the
reaction layers is not applicable, because single electron captures are
blocked, so that the neutron fraction is fixed, and therefore neutrons are not
an independent component of the crust matter. The absence of neutron diffusion
in the shells between the reaction layers, stems from the constancy of the
neutron fraction (concentration) in these shells. In the reaction layers, the
outward force resulting from neutron fraction gradient is balanced by the
inward gravitational force acting on unbound neutrons. Neglecting the thickness
of the reaction layers compared to the shell thickness, we obtain condition
$e^{\Phi(r)}f_Q(r)g(r)=$constant, where $g$ is Gibbs energy per nucleon,
undergoing discontinuous drops on the reaction surfaces, and
$f_Q(r)g(r)={\widetilde{g}}(r)$ is a continuous function, due to the factor
$f_Q(r)$ canceling the discontinuities (drops) in $g(r)$. The function $f_Q(r)$
is calculated using the Tolman-Oppenheimer-Volkov equations from $f_Q(P)$ and
$g(P)$ obtained from the equation of state (EOS) with discontinuites. The
constancy of of $e^{\Phi(r)}{\widetilde{g}}(r)$ is an extension of the standard
relation $e^{\Phi(r)}{{\mu}}_{\rm b}(r)=$constant, valid in hydrostatic
equilibrium for catalyzed crust.
