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

Chapter 44.2.3 Sulphur sources and SO 2 productionTable 4.5 gives an overview of the major sulphur sources for the manufacture of H 2 SO 4 andsome characteristic features of the gas containing SO 2 arising from these sources.4.2.3.1 Sulphur combustionElemental sulphur is derived from desulphurisation of natural gas or crude oil by the Clausprocess. Elemental sulphur is delivered to the plant preferably liquid but also solid (attemperatures of 140 – 150 °C) and, if necessary, filtered prior to combustion.The combustion is carried out in one-stage or two-stage units between 900 and 1500 °C. Thecombustion unit consists of a combustion chamber followed by a waste heat boiler. The SO 2content of the combustion gases is generally as much as 18 % v/v and the O 2 content is low (buthigher than 3 %). The inlet gases content to the conversion process is generally between 7 – 13% SO 2 , if necessary adjusted by dilution with air.4.2.3.2 Regeneration of spent acidsSpent acids arise from processes where H 2 SO 4 or oleum is used as a catalyst (alkylation,nitration, sulphonation, etc.) or from other processes where H 2 SO 4 is used to clean, dry, oreliminate water.The thermal decomposition of spent sulphuric acids to sulphur dioxide is carried out underoxidative conditions in a furnace at a temperature of about 1000 °C. The process is described bythe following equation:H 2 SO 4 SO 2 + H 2 O + ½O 2H = + 202 kJ/moleSpent acids are atomised into small droplets to achieve good thermal decomposition. Therequired energy is provided by the injection of hot flue-gases. The additional volume leads todilution of the SO 2 content. This dilution can be reduced by preheating of the combustion air orby enriching with O 2 .The reductive decomposition of spent sulphuric acid to sulphur dioxide is carried out byheating the spent sulphuric acid and contained residues in a rotary kiln on a coke bed totemperatures of 400 – 1000 °C. The spent sulphuric acid is decomposed to SO 2 . Organiccompounds are partially reduced to coke but contribute, due to the reductive conditions, as COand VOCs to the exhaust gas. Inorganic materials, such as Mg, Fe, Pb and heavy metalcompounds remain in the coke. The exhaust gases are treated by thermal oxidation attemperatures of 1100 – 1300 °C with sufficient residence times.The SO 2 content in the combustion gases depends mainly on the composition of the spent acids(content of water and organics), and it can vary from 2 to 15 %. Sulphur can be fed to adjust theSO 2 content and to minimise variations. Most of the energy from the combustion gases isrecovered as steam in a waste heat boiler. The exhaust gases are cleaned downstream, demisted,dried and reheated before passing to the converter.Another option is the production of liquid SO 2 or sodium bisulphite as a co-product of thesulphuric acid plant.156 Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers