Sulphur and Sulphuric acid · Uses of Sulfuric Acid •Fertilizer, and leather production...

1 1 CHEMICAL PROCESS INDUSTRIES

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

CHEMICAL PROCESS INDUSTRIES

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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)


11 CHEMICAL PROCESS INDUSTRIES

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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)



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


Gas entering fourth pass 427

Gas leaving fourth second pass 430


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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.


Sulphur and Sulphuric acid · Uses of Sulfuric Acid •Fertilizer, and leather production...

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