Research on Production of ¹⁵N Isotope with N₂ by Gas Diffusion Method

There are two natural stable isotopes of nitrogen, ¹⁴N and ¹⁵N, with isotopic abundances of 99.636% and 0.364%. ¹⁵N is widely used in research and analysis in atmospheric tracing, agricultural and forestry research, and origin tracing. Nitrogen (N₂) is an ideal medium for separating ¹⁵N due to its low price and stable physical and chemical properties. The gas diffusion method boasts high flow rates and reliability. For gases with relatively small molecular weights such as N₂, the gas diffusion method can achieve higher separation coefficients. The advancement of polymer membrane can significantly reduce the equipment costs of the gas diffusion process, leading to better efficiency of gas diffusion separation. The Institute of Technical Physics at Tsinghua University selected appropriate organic polymer membranes, and made it into multi-layer tubular membranes. Tubular membranes are used into manufacture separators, which are manufactured into the gas diffusion separation of various stable isotopes. A four-stage diffusion cascade was designed and constructed, and separation experiments for multiple isotopes such as ¹³C and ²⁸Si were conducted on this cascade in Institute of Technical Physics. High-speed maglev compressors were applied in the four-stage cascade for gas pressurization. The compressors were independently developed by the Institute of Technical Physics, and have demonstrated good performance in practical applications. Based on the preliminary work of Institute of Technical Physics, single-stage and four-stage diffusion experiments using N₂ as the medium to separate ¹⁵N were conducted to investigate the fluid performance parameters, separation coefficient, etc. In single-stage diffusion experiments, the pressure ratio across the membrane and the split ratio were controlled by adjusting valves, and in four-stage diffusion experiments, which was controlled by adjusting the speed of maglev compressors, and the split ratio was controlled by adjusting the reflux flow of the first stage and the fourth stage. The experimental results show that when controlling the feed pressure at 2.67 kPa and pressure ratio across the membrane at 4.2, the overall separation factor of the single-stage platform could reach 1.016. When controlling the feed pressure at 2.69 kPa and pressure ratio across the membrane at 3.9, the overall separation factor of the four-stage cascade could reach 1.057. The experimental results prove that it’s feasible to use N₂ as a medium to separate ¹⁵N isotope by gas diffusion. The follow-up plan is to expand the scale of cascade to explore the economic feasibility of separating ¹⁵N using N₂ as a medium.