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

Chapter 44.2 Applied processes and techniques4.2.1 OverviewFor an overview of the production of H 2 SO 4 , see Figure 4.3. H 2 SO 4 is produced from SO 2 ,which is derived from various sources (see Section 4.2.3), such as combustion of elementalsulphur or roasting of metal sulphides. SO 2 is then converted into SO 3 in a gas phase chemicalequilibrium reaction, using a catalyst:SO 2 + ½ O 2 d SO 3jH 0 = -99 kJ/moleThe conversion rate is defined as follows:SO 2 in – SO 2 outConversion rate =SO 2 inx 100 (%)Both thermodynamic and stoichiometric considerations are taken account of by maximising theformation of SO 3 . The Lechatelier-Braun principle, which states that when an equilibriumsystem is subjected to stress, the system will tend to adjust itself in such a way so as to partlyrelieve the stress, needs to be taken into account for optimisation of the equilibrium. Thestresses are, for instance, variations of temperature, pressure, or the concentration of a reactant.For SO 2 /SO 3 systems, the following methods are available to maximise the formation of SO 3 :• as this is an exothermic process, a decrease in temperature by removal of the heat willfavour the formation of SO 3• increased oxygen concentration• SO 3 removal (as in the case of the double absorption process)• increased pressure• catalyst selection, to reduce the working temperature (equilibrium)• longer reaction time.SulphursourceSO 2productionEmissionto airAir or oxygenOptional:purificationOptionalConversionSO 2 to SO 3Tail gastreatmentOptionalH 2OAbsorptionSO 3 in H 2 SO 4Optional:dilutionpurificationH 2SO 4oroleumFigure 4.3: Overview of the production of H 2 SO 4This figure is based on [58, TAK-S, 2003]Optimising the overall system behaviour requires a balance between reaction velocity andequilibrium. However, this optimum also depends on the SO 2 concentration in the raw gas andon its variability. Consequently, each process is more or less specific for a particular SO 2source.Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers 151