Background: The close boiling points of carbon tetrafluoride (CF4) and nitrogen trifluoride (NF3) and the chemical inertness of CF4 make it difficult to remove it from NF3. Methods: A method for cleaning NF3 from CF4 at the gas-liquid interface is proposed. The purification is carried out by a sequence of cycles: (1) fractional condensation of NF3 at a cryostat temperature (T-cry), at which the equilibrium pressure of CF4 exceeds its partial pressure in the feed mixture; and (2) pumping out the gas phase enriched with CF4 and high-boiling impurities. The cycles are repeated at a next T-cry corresponding to the new CF4 content, and are completed by cryogenic distillation of the condensate with the removal of low-boiling impurities. Findings: The CF4 content of 5000 ppm in a 25 g NF3 sample was reduced to less than 100 ppm in three cycles at T-cry = 120 K; a CF4 separation coefficient above 40 was achieved. The method can be used for the industrial production of high-purity NF3, with the cryostat refrigerant being the only consumable material, as well as for the separation of other substances with close boiling points, such as isomers or isotopes. (C) 2021 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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