Simulating condensation of ammonium fluoride during fluorination of yttria

Abstract

The preparation of yttrium fluoride (YF₃) by fluorination of yttria with ammonium bifluoride is an established process. During fluorination, large amounts of ammonium fluoride (NH₄F) vapours are generated in the reaction zone. In order to prevent back reaction, the vapours must be suitably removed from the reaction zone. In usual practice, gas purging (argon or dry air), at fairly high flowrate, removes the vapours suitably. The high flowrates have a considerable effect on the temperature inside the reactor. The ammonium fluoride vapours, transported with the inert gas, condense on the cooler regions of the reactor. The condensed vapours being highly corrosive attack the reactor wall. The present study attempts to address the above issue. A three-dimensional numerical heat transfer model has been developed for the fluorination reactor using a commercial software – Fluent 6·1. The simulations were attempted with varying argon flow velocities, to identify the heat taken away by argon during the reaction. When surface cooling was applied, condensation of ammonium fluoride vapours occurred on the inner surface of the walls irrespective of the argon flow velocity; the simulated results for the extent of condensation from the exit end of the reactor were in fair agreement with the corresponding experimental results.