Sulphur and Sulphuric acid 1
Sulfur Most important and basic material Exists in nature both in free state and combined in ores; Important constituent of petroleum Important constituent of natural gas Pyrite FeS 2 Sphalerite ZnS Chalcopyrite CuFeS 2 2
Applications Majorly in the production of; Sulfuric acid (up to 90%) Wood pulp Carbon disulfide Insecticides Fungicides Bleaching agents Vulcanized rubber Detergents Pharmaceuticals 3
Sulfur Manufacturing Process 1. Frasch Process (from sulfur bearing rocks) 2. Claus Process (from H2S in natural gas, coke oven gas, petroleum refinery gas etc.) 4
Frasch Process The Frasch Process uses three concentric pipes driven into the ground. Superheated steam (hot water at about temperature of 160 C) is pumped under pressure through the outermost pipe into the sulfurbearing-rock formation. This heats the rock above the melting point of sulfur, 119 o C. The molten sulfur is heavier than water and collects in a pool. Heated, compressed air pumped through the innermost pipe works the sulfur in the pool into a froth that rises to the surface through third pipe. 5
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Claus Process 7
Forms of sulfuric acid Although 100% sulfuric acid can be made, this loses SO3 at the boiling point to produce 98.3% acid. The 98% grade is also more stable for storage, making it the usual form for "concentrated" sulfuric acid. Other concentrations of sulfuric acid are used for different purposes. Some common concentrations are: 10%, dilute sulfuric acid for laboratory use 33.5%, battery acid (used in lead-acid batteries) 62.18%, chamber or fertilizer acid 98%, concentrated 8
Uses of Sulfuric Acid Fertilizer, and leather production Dyeing of fabrics Paper production Ore processing Wastewater processing Nitration production of explosives Acid batteries Dehydrating agent 9
CONTACT PROCESS CHEMISTRY In the first step, sulfur is burned to produce sulfur dioxide. S(s) + O2(g) SO2(g) This is then oxidised to sulfur trioxide using oxygen in the presence of a vanadium(v) oxide catalyst. 2 SO2 + O2(g) 2 SO3(g) (in presence of V2O5) Finally the sulfur trioxide is treated with water (usually as 97-98% H2SO4 containing 2-3% water) to produce 98-99% sulfuric acid. SO3(g) + H2O(l) H2SO4(l) Directly dissolving SO3 in water is impractical due to the highly exothermic nature of the reaction. Mists are formed instead of a liquid. Alternatively, the SO3 is absorbed into H2SO4 to produce oleum (H2S2O7). H2SO4(l) + SO3 H2S2O7(l) Oleum is reacted with water to form concentrated H2SO4. H2S2O7(l) + H2O(l) 2 H2SO4(l) 10
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Material Processing Conversion of SO 2 into SO 3 The design and operation of sulphuric acid plants are focused on the following gas phase chemical equilibrium reaction with a catalyst:- SO2 + ½ O2 < -> SO3 This reaction is characterized by the conversion rate, which is defined as follows:- conversion rate = (SO2)in (SO2)out x 100(%) (SO2) in 12
Conversion of SO 2 into SO 3 (cont.) The Lechatelier Principle is usually taken into account in deciding how to optimise the equilibrium. For SO2/SO3 systems, the following methods are available to maximise the formation of SO3 :- Removal of heat a decrease in temperature will favour the formation of SO3 since this is an exothermic process Increased oxygen concentration Removal of SO3 (as in the case of the double absorption process) Raised system pressure Selection of the catalyst to reduce the working temperature (equilibrium) 13
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Location Temperature O C Equivalent Conversion, % Gas entering first pass 410 Gas leaving first pass 602 Rise in temperature 192 74 Gas entering second pass 438 Gas leaving second pass 485 Rise in temperature 47 18.4 Gas entering third pass 432 Gas leaving third pass 443 Rise in temperature 11 4.3 Gas entering fourth pass 427 Gas leaving fourth second pass 430 Rise in temperature 3 1.3 16
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Absorption of SO3 Sulphuric acid is obtained from the absorption of SO3 and water into H2SO4 (with a concentration of at least 98%). The efficiency of the absorption step is related to:- The H2SO4 concentration of the absorbing liquid (98.5-99.5%) The range of temperature of the liquid (normally 70 C-120 C) The mist filter The temperature of incoming gas The co-current or counter-current character of the gas stream in the absorbing liquid 18
Process description( The single absorption process) In the contact process, elemental sulfur is melted, filtered to remove ash, and sprayed under pressure into a combustion chamber. The sulfur is burned in clean air that has been dried by scrubbing with 93 to 99 percent sulfuric acid. The gases from the combustion chamber cool by passing through a waste heat boiler and then enter the catalyst (vanadium pentoxide) converter. Usually, 95 to 98 percent of the sulfur dioxide from the combustion chamber is converted to sulfur trioxide, with an accompanying large evolution of heat. After being cooled, again by generating steam, the converter exit gas enters an an oleum tower that is fed with 98 percent acid from the absorption system. The gases from the oleum tower are then pumped to the absorption column where the residual sulfur trioxide is removed. 19
Dual Absorption Process In the dual absorption process the SO3 gas formed in the primary converter stages is sent to an interpass absorber where most of the SO3 is removed to form H2SO4. The remaining unconverted sulfur dioxide is forwarded to the final stages in the converter to remove much of the remaining SO2 by oxidation to SO3, whence it is sent to the final absorber for removal of the remaining sulfur trioxide. 20
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