15 N separation in the Nitrox system under pressure

The basic isotope exchange reaction responsible for the separation of 15 N in Nitrox system is that between gaseous nitrogen oxides and aqueous nitric acid with single stage separation factor α = 1.055 for M.l-1 nitric acid, at 25 deg. C and atmospheric pressure. The rate of nitrogen isotope exchange between NO and HNO3 has been measured as a function of nitric oxide pressure 0.1 - 0.4 MPa for 1 and 2 M.l-1. It is concluded that 15 N/14 N exchange rate in NO-HNO3 system has a linear dependence on NO pressure as indicated by rate measurements at different NO partial pressure and constant overall pressure, by adding helium in reactor. Using the rate law: R = [HNO3]2 [N2O3] the 15 N/14 N exchange rates for nitric acid concentrations 1.5 - 10 M.l-1 were calculated. In order to know what happens in 15 N separation at higher pressure, when the isotopic transport between two phases is improved, a stainless steel laboratory experimental setup with 1000 mm long x 18 mm i,d. column, packed with triangular wire springs 1.8 x 1.8 x 0.2 mm was utilised. At 0.15 MPA and 2.36 ml.cm-2 . min-1 flow rate HETP was 7% smaller than at atmospheric pressure and 1.5 times smaller flow rate. HETP at 3.14 ml . cm-2 . min-1 flow rate and 0.18 MPa is practically equal with that obtained at atmospheric pressure and 2 times smaller flow rate. The operation of the 15 N separation setup at 0.18 MPa, instead of atmospheric pressure, will permit to double the 10 M.l-1 nitric acid flow rate and of 15 N production of the given column. (authors)