NH3 is oxidised with air on catalyst to form NO at high temperature (700 – 1000 °C ), in a quick exothermic reaction. After cooling, NO is oxidised by excess air to give NO2. This oxidation continues into the absorbtion stage by H2O, where NO2 is transformed into HNO3.
Platinum (PT) catalyst is lost ( especially in high pressure design plants ), entrained from the converter ; Alternatively, where a Palladium (PD) gauze is installed to collect lost PT, PD can be displaced and lost as oxide.
Filter installation is best done downstream of the waste heat boiler.
Recovery of precious metals dust.
High Efficiency, Brownian Diffusion ‘Becofil’ Candle Filters, hanging type HT(2) but installed horizontally, with TGW15 or B14W Glass Fibre elements and 304 Stainless Steel structure if temperature is < 350 °C, or 321 Stainless Steel structure if temperature is > 350 °C.
The precious metal solids will be collected in the dense fibre bed. The degree of blockage increases up to the designed resistance limit.
To allow continuous operation even when filters are removed for recovery of metals, 2 sets of filters can be installed in parallel units.