  The simplest model for non-congruent phase transition of gas-liquid type was
developed in frames of modified model with no associations of a binary ionic
mixture (BIM) on a homogeneous compressible ideal background (or non-ideal)
electron gas /BIM($\sim$)/. The analytical approximation for equation of state
equation of state of Potekhin and Chabrier of fully ionized electron-ionic
plasma was used for description of the ion-ion correlations (Coulomb
non-ideality) in combination with ``linear mixture'' (LM) approximation. Phase
equilibrium for the charged species was calculated according to the
Gibbs-Guggenheim conditions. The presently considered BIM($\sim$) model allows
to calculate full set of parameters for phase boundaries of non-congruent
variant of phase equilibrium and to study all features for this non-congruent
phase transition realization in Coulomb system in comparison with the simpler
(standard) forced-congruent evaporation mode. In particular, in BIM($\sim$)
there were reproduced two-dimensional remarkable (``banana-like'') structure of
two-phase region $P-T$ diagram and the characteristic non-monotonic shape of
caloric phase enthalpy-temperature diagram, similar to the non-congruent
evaporation of reactive plasma products in high-temperature heating with the
uranium-oxygen system. The parameters of critical points (CP) line were
calculated on the entire range of proportions of ions $0<X<1$, including two
reference values, when CP coincides with a point of extreme temperature and
extreme pressure, $X_{T}$ and $X_{P}$. Finally, it is clearly demonstrated the
low-temperature property of non-congruent gas--liquid
transition---``distillation'', which is weak in chemically reactive plasmas.
