sulphur and sulphuric acid · uses of sulfuric acid •fertilizer, and leather production...
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Sulphur and Sulphuric
acid
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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 FeS2
• Sphalerite ZnS
• Chalcopyrite CuFeS2
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Applications
• Majorly in the production of; • Sulfuric acid (up to 90%)
• Wood pulp
• Carbon disulfide
• Insecticides
• Fungicides
• Bleaching agents
• Vulcanized rubber
• Detergents
• Pharmaceuticals
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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.)
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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 sulfur-
bearing-rock formation. This heats the rock above the melting point of
sulfur, 119oC. 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.
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Claus Process
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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
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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
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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)
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Material Processing
Conversion of SO2 into SO3
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
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Conversion of SO2 into SO3(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)
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Location Temperature OC 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
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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
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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.
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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.
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