Large Volume Inorganic Chemicals - Ammonia ... - ammk-rks.net

Chapter 44.4.2 Double contact/double absorption processDescriptionIn the double contact process, a primary SO 2 conversion of 85 – 95 % is achieved in the firstcatalysis stage of the converter before entry into an intermediate absorber, depending on thearrangement of the converter beds and the contact time. After cooling of the gases toapproximately 190 ºC in a heat exchanger, the SO 3 already formed is absorbed in 98.5 – 99.5 %sulphuric acid. If necessary, an oleum absorber is installed upstream of the intermediateabsorber. The absorption of SO 3 shifts the reaction equilibrium significantly towards theformation of more SO 3 . This results in a considerably higher conversion rate, if the residual gasis passed through the following converter beds (usually one or two). The SO 3 which is formedin the second catalysis stage is absorbed in the final absorber. Figure 4.11 shows the example ofa 2+2 double contact double absorption plant. 2+2 indicates the number of catalyst beds beforeand after intermediate absorption. 3+1 represents the alternative configuration with 4 beds, 3+2is the normal configuration with 5 beds (see Section 4.4.3).SulphurSteamSulphurburnerWaste heat boilerDried airH 2O1CatalystbedsHeatexchanger2Tail gasHeatexchanger3CandlefilterIntermediateAbsorptionHeatexchangerHeatexchanger4FinalAbsorptionH 2SO 4ProductH 2SO 4H 2SO 4ProductH 2SO 4Figure 4.11: Example for a 2+2 double contact/double absorption processThis figure is based on [57, Austrian UBA, 2001].Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers 173