Differential equations describing open crystallization processes, which occur when a solvent is distilled under isothermal-isobaric conditions, are derived. Such processes are shown to be topologically isomorphic to isothermal or isobaric open vaporization and isobaric open crystallization from melts. The behavior of isotherms, isobars, and isopotential curves of the solvent in the nearest vicinity of singular points is considered for the two-phase equilibrium in a solution-solid state system.