The equations, describing the processes in the rectification units have been analyzed. The concentration field in a condenser-evaporator and its influence on the boiling pressure has been investigated. The concept of rectification function has been formulated and the methods of the definition of boundary conditions have been examined. Requirements to the quantity of initial parameters have been defined. Factors of influence of the initial values on the finite data have been calculated. Calculation algorithms have been generated and the convergence of algorithms has been investigated at different methodical descriptions of rectification. The control calculations have been carried out and the reasons of unbalance results have been established. It is shown the necessity of providing with full internal co-ordination in the creation of the expanded model including both a column and the condenser-evaporator. Metering characteristics of rectification unit for nitrogen manufacturing have been calculated on the base of expanded model. These characteristics are suitable for quality control of production nitrogen.