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Kristian Viegaard Raun - Guest lecturer

Formaldehyde (CH2O) may be synthesized industrially by selective oxidation of methanol over an iron-molybdate (Fe-Mo) oxide catalyst according to: CH3OH + ½O2 → CH2O + H2O. The reaction is normally carried out in a multitubular reactor (tube length = 1-1.5 m) with stoichiometric excess of oxygen (10% MeOH, 10% O2, 5 % H2O, balance N2) at atmospheric pressure and 270 400 °C (overall plant yield = 88-92 %), known as the Formox process [1]. The fresh catalyst consists of two phases Fe2(MoO4)3 and MoO3. Fe2(MoO4)3 is the active phase and excess MoO3 must be present to form a surface layer of excess MoOx and make the pellets mechanically stronger. Pure MoO3 in itself has low activity.
However, under the reaction conditions molybdenum oxide forms volatile species with methanol in the feed gas and migrate through the catalyst bed. The volatile molybdenum species decompose and accumulate downstream in the reactor bed leading to significant pressure drop build-up and finally plant shutdown. The volatility of the Mo in the catalyst and pressure drop build-up in the industrial reactor is the main reason for the short lifetime of only 1–2 years depending on the operating conditions [2]. In this work the rate of volatilization of Mo from industrial catalyst pellets has been studied as a function of operating conditions and a single pellet model that take the relevant phenomena into account has been developed.
22 May 2018

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ID: 154731529