DAMIAN AXENTE, STEFAN BUGEAC, STEFAN GERGELY, MIHAI GLIGAN, CRISTINA MARCU, ZSOLT SZUCS, CODRUTA VARODITHE PRODUCTION OF THE ISOTOPE 15N BY ISOTOPIC EXCHANGE IN NITROX SYSTEM AT PRESSURE The operation of 15N production plant at pressure of 0.7 +/- 0.05 bar in stable conditions at total reflux and in production is demonstrated. The 10M HNO3 flows used in the primary column were 6.6 L/h and 6.0 L/h, 47, respectively 33% higher than that of atmospheric pressure (4.5 L/h) and corresponding flow rates of the same column were 3.12 mL/cm2. min, respectively 2.83 mL/cm2.min for operation at pressure, compared to 2.12 mL/cm2’.min at atmospheric pressure. The 10M HNO3 flows used in the final column were 324 mL/h and 264 mL/h, 62, respectively 32 % higher than that of the atmospheric pressure (200 mL/h) and flow rates of that column were: 3.05 mL/cm2.min, respectively 2.49 mL/cm2.min, compared to 1.86 mL/cm2’.min for atmospheric pressure operation. The HETP values are lower for both separation columns operated at pressure and higher 10M HNO3 flow rates, that being an argument in the favour of operation the 15N production plant by isotopic exchange in Nitrox system at pressure. The nitrogen enriched in 15N losses in the waste sulphuric acid, evacuated at the bottom of product refluxer stage II, were : 5.18 -10.58 ppm of feeding with 10M HNO3 of the plant. Any losses of nitrogen enriched in 15N represent an additional uncontrolled production, which diminishes the production of 15N plant. In the case of operation at pressure the flows of H15NO3 extracted as plant product are higher than those corresponding to atmospheric pressure because the nitric acid solution is chemically equilibrated with nitrogen oxides at the operation pressure and temperature. At a pressure of 1 atm. in the 15N production plant the nitrogen content of nitric acid solution would be 10.919M instead of 10M.