FRACTIONATION OF¹⁵N IN COUNTERCURRENT HNO₃-NO SYSTEM AT LOW TEMPERATURE

Abstract

This article describes the method for fractionation of 15 N isotope based on the exchange of nitrogen isotopes in the system consisting of liquid mixtures of nitric acid and nitrogen oxides and the gaseous nitrogen oxide in the temperature range between 233 and 263 K under atmospheric pressure developed by the authors. The chemical interactions occurring between liquid nitric acid and gaseous nitrogen oxide at low temperatures and the equilibrium of the nitrogen isotopes exchange has been studied by means of the analysis of each co-existing phase in a packed column with counter-current movement of liquid and gas phases. The results obtained by theoretical modeling and experimental testing of the experimental plant have allowed one to conclude that the low temperature NITROX method is promising for the large-scale 15 N production. This method improves upon the main technological parameters of the process as compared to traditional room temper-ORDER REPRINTS ature NITROX system. It allows for increasing the productivity of the same column and the degree of 15 N extraction from the feed nitric acid up to two times, and for decreasing almost twice the consumption of sulphur dioxide (which is used to convert HNO 3 to NO). ORDER REPRINTSThe NITROX separation columns usually operate at the atmospheric pressure and room temperature of ϳ298 K, which has been indicated by Krell et al. (17) as the optimal one. However, the authors of other early work (16) determined the optimal temperature to be 318 K, because at this temperature the composition of the gaseous mixture of nitrogen oxides in the column was identical to that of the gas flow from the refluxer. Later systematic studies (24-26) of influence of the temperature on the 15 N enrichment have shown that the optimum temperature is close to 318 K. Khoroshilov and Katalnikov (25,26) have also found that an increase of temperature from 283 K to 318 K results in ϳ 1.2-1.4 fold decrease of SO 2 expenditure at a constant feed-flow of the initial nitric acid. However, they did not consider the influence of temperature on the degree of 15 N extraction from the initial nitric acid and on the productivity of separation set-up.Simultaneous optimization of both the degree of extraction and enrichment in isotope separations along with minimization of the production costs were considered in (27)(28)(29)(30)(31)(32) where the idea to use non-isothermal conditions in separation column was also originated. It was suggested to maintain a certain temperature gradient, which gradually increased along the column toward the refluxer. A lower temperature at the feed-point of the column provided a high degree of extraction of the target component from the initial mixture due to a high value of separation factor. At the same time a gradual increase of temperature in direction to the refluxer end decreased the HETS values in the main part of the column resulting in an increase of enrichment at the refluxer end of the column. The authors of this elegant idea investigated its applicability for s...