Separation characteristics of trace helium-3 in a continuous 2 K cryogenic distillation system: experimental and numerical investigation

The separation of 3 He from natural helium resources is challenging due to its ultra-low abundance and the similar properties of the two isotopes. Cryogenic distillation offers a promising route, yet its application to trace-level mixtures remains poorly understood. In this work, a continuous cryogenic distillation system operating at approximately 2 K was constructed to separate trace 3 He from commercial 4 He containing ∼1.5 × 10 −8 of 3 He. Under total reflux mode, the separation performance improves as the operation approaches the He-II regime ( T < 2.176 K), where the height equivalent to a theoretical plate (HETP) reaches a minimum of approximately 10 cm. Cooling further into the deeper He-II regime did not improve enrichment. No flooding was observed below 4 cm/s. The HETP values are higher than those reported for high-concentration systems, reflecting the inherently weak mass transfer driving force at ultra-low 3 He concentrations. Under total recycle mode, continuous operation achieved simultaneous top enrichment and bottom depletion. The maximum top enrichment factor ( E top ) reached 168, with a bottom depletion factor ( E bottom ) of 51 and a 3 He recovery of 98%. The bottom 3 He/ 4 He ratio was reduced to 2.9 × 10 −10 , meeting the requirements for ultra-high-purity 4 He. Parametric analysis using a validated dynamic model further revealed that the withdrawal ratio (W) has a stronger impact on E top than reboiler heating power (Q), with lower W values significantly enhancing enrichment. This study provides valuable experimental evidence and technical guidance for the separation of trace helium isotopes, and highlights its potential for engineering applications.