ДОСЛІДЖЕННЯ КІНЕТИКИ ОКИСНЕННЯ МОНООКСИДУ АЗОТУ ПРИ ЗНАЧНОМУ НАДЛИШКУ КИСНЮ

This paper addresses the rate-limiting stage of nitric acid production under a single operating pressure of 0.718 MPa (7.3 atm) — the homogeneous gas-phase oxidation of NO to NO₂. A review of the literature reveals that the third-order rate equation (second order with respect to NO, first order with respect to O₂) is predominantly employed to describe the non-catalytic reaction rate. At the same time, catalytic systems with kinetic equations of different order have been proposed for exhaust gas treatment applications. The objective of the present work was to examine the feasibility of describing NO-to-NO₂ oxidation by a second-order kinetic equation for NO concentrations characteristic of industrial nitric acid synthesis conditions, adopting zero order with respect to oxygen when the latter is present in excess. In the first stage of calculations, six process units and interconnecting pipelines of a 15 t/h, 56–58% nitric acid plant — spanning the interval from the waste-heat boiler to the absorption column over the temperature range 318–610.5 K — were selected as objects of study. For each unit, the reaction rate and residence time were computed from the third-order equation. The resulting rate values were then used to evaluate the second-order rate constant k₂, assuming zero order in oxygen. Residence times τ₂ were subsequently determined as a function of the required NO oxidation degree using the integrated form of the second-order equation. Comparison of τ₂ with τ demonstrated that the second-order equation can, in principle, be employed for process design calculations in units where the ratio of initial oxygen to NO concentrations exceeds 1.75. An analogous calculation was performed for the absorption column, in which a substantial oxygen excess is maintained by injection of supplementary air. Using eight trays distributed over different heights of the column in the temperature range 303–324 K, the second-order equation with a mean rate constant (10.6) was shown to describe the oxidation rate with full adequacy. Adoption of the second-order equation in place of the conventional third-order equation considerably simplifies calculations of either the oxidation time at a specified degree of conversion or the degree of conversion at a specified oxidation time.