C H E M I C A L P R O C E S S T E C H N O L O G Y

C H E C 3 2 2

SULFUR & SULFURIC ACID

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Sulfur (S)

INTRODUCTION :

1. Basic raw material for production of sulfuric acid.

2. Properties

o Atomic weight 32. 07.

o Bright yellow crystalline solid.

o Physical state : (Solid) pure element and as sulfide and sulfate minerals.

3. Specific gravity- 1.803, insoluble in water, soluble in organic solvents.

4. Uses

o 80-90 % used for sulfuric acid manufacture.

o Precursor to other chemicals (sulfites and sulfates)

o Direct use for vulcanization of rubber

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Process Selection

CLASSIFICATION OF MANUFACTURING PROCESS

1. Frasch process: Elemental sulfur mining

2. Claus Process: Oxidation – reduction of hydrogen sulfide

3. Finnish process: elemental sulfur from pyrites

INDIAN SCENARIO

India is practically devoid of deposits of elemental sulfur.

Increasing demand of natural gas has given a boost to the sulfur-recovery

aspect of natural gas.

Petroleum refinery stream is also a source of H2S.

Iron pyrite (sulfides of iron) availability : Important deposits of pyrites are

those of Amjhore in Rohtas (Bihar).

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SULFUR FROM SOUR NATURAL GAS (CLAUS PROCESS)

OXIDATION-REDUCTION OF H2S

Raw material :

H2S from natural gas and petroleum refinery

AMINE ABSORPTION PROCESS FOR HYDROGEN SULFIDE REMOVAL

Sulfur recovery from sour natural gas is

conducted in two stages:

[1] Scrubbing the gas with an amine

solution (Methyl-diethanolamine MDEA)

[2] Recovery of solvent (by indirect heating)

* Process is known as ‘sweetening’ of

Natural gas.

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Chemical reactions

[1] First reaction: simple combustion of H2S stream in air, carried

out in waste heat boiler to capture heat evolved as steam.

2H2S + 3O2 2SO2 + 2H2O; ΔHo = -247.89 Kcal

[2] Second reaction (catalytic)

4H2S + 2SO2 6S (v) + 4H2O; ΔHo = - 42.24 Kcal

100

Catalyst: Al2O3 (Alumina) / Iron Oxide H2S

% conversion

Equilibrium curve 50

100 300 500 T (˚C)

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Process flow sheet

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Major engineering problems

1. Two stage reactor design for exothermic SO2 oxidation

of H2S. 70-80 % conversion in first stage at 300-400 C

range followed by 250-300 C operation in second

reactor to obtain favorable equilibrium.

2. Heat exchange for molten sulfur handling.

3. Corrosion

4. Final clean up of stack gases.

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Possible solutions

Multistage operation

If the sulfur vapor from the one stage is condensed out, and the residual gases are blended with further hydrogen sulfide and passed over catalyst, the equilibrium conversion of 94-95% can be obtained.

Effluent gas from a single stage may contains 2-3% SO2, which would represent both loss of feedstock and emission problem.

Need to look at equilibrium data and catalyst for deciding temperature range.

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SULFURIC ACID(H2SO4)

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Properties Highly corrosive strong

mineral acid, molecular wt : 98, MP :10.5 oC, BP : 340oC

Colorless to slightly yellow viscous liquid

Soluble in water at all concentrations.

The corrosiveness of it is mainly due to its strong acidic nature, strong dehydrating property and its concentrated strong oxidizing property.

Commercially produced in

various acid strength 33.33 to 114.6 wt% Strength over 100% referred as

‘Oleum’

Applications Lead acid batteries for cars

and other vehicles Mineral processing Fertilizer manufacturing Oil refineries Wastewater processing Chemical synthesis.

CHAMBER PROCESS CONTACT PROCESS

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Developed first, produced acid of concentration less than 80%

Used a homogeneous catalyst (nitrogen oxide)

Used for producing sulfuric acid

Virtually obsolete : corrosion of lead, absorption of NO2 gas

Yield 98% H2SO4 and higher

Originally used a supported platinum catalyst, but later replaced by a supported vanadium catalyst

Far more economical process for producing sulfur trioxide and concentrated sulfuric acid

Current process with some advances (DCDA)

Classification of Processes

CONTACT PROCESS

Reactions:

[1] Sulfur is burned to produce sulfur dioxide and heat.

S(s) + O2 (g) → SO2 (g) ∆H = -70.9 kcal

** Sulfur dioxide may also be obtained via oxidation of pyrites or from other smelter source. Dust removal equipment add the capital cost for these secondary sources.

[2] The second reaction, oxidation of sulfur dioxide to sulfur trioxide with air (less exothermic, equilibrium reaction)

V2O5

SO2 (g) + ½ O2 (g) SO3 (g) ∆H = -23 kcal

[3]Adsorption of formed SO3 in water

SO3(g) + H2O (l) H2SO4 (l) ∆H = -31 kcal

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Catalyst: (Speeds up the reaction but not consumed in reaction)

Formation of sulfur trioxide is catalyzed by 6-10% vanadium pentaoxide (V2O5) coating.

Platinum would be a more suitable (more active) catalyst, but it is very costly and easily poisoned.

Supported on a powdered or porous carrier Provide large surface area

Resistance to process gases at high temperature

Examples- alumina, silica gel, kieselghur, pumice.

Promoted by alkali and/or metallic compounds Example-potassium hydroxide

Below 400 °C the V2O5 is inactive as a catalyst, and above 620 °C it begins to break down

Catalyst deactivation: Life of modern vanadium/potassium catalyst : 5-20 years

Purification of air and SO2 is necessary to avoid catalyst poisoning (ie. removing catalytic activities). The gas is then washed with water and dried by H2SO4

Regular screening of catalyst ( 1 year). Temperature limitation.

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Thermodynamics & Kinetics Considerations: • The equilibrium for a gas phase reaction can be expressed in partial pressures:

Kp=PSO3/(PSO2 . P1/2O2)

• At 400C, equilibrium conversion is 96%. But the time required for reaction (2nd) is long, which require a large reactor and catalyst volume. • For process optimization

•Multistage bed (3 or more catalyst bed) •Starting with very fast reaction rates at 550-600 C with 60-65% conversion. •Cooling of gas mixture at 400-450 C, for more favorable equilibrium •Pass over three or more beds to reached about97-98% conversion to SO3.

• Air can be added at this stage to assist in displacing the equilibrium further right. • It is expected under pressure equilibrium of the reaction more to the right

•But it has not worth the additional capital cost required at high pressure •Corrosion problem

T(C) 400 500 600 700 800 900

Kp 397 48.1 9.53 2.63 0.915 0.384

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Process Description 1. Burning an atomized jet of filtered molten sulfur in a stream of dry air (7-10 % SO2

and 11-1 4% O2). 2. A fire tube boiler is used to reduce gas temperature to the 400 to 240 C range,

simultaneously generating steam. 3. Next gas mixture sent to first catalyst bed of two-stage catalytic converter with 80%

conversion and raising temperature upto 600 C. 4. The conveter product is cooled to 300 C and fed to second stage, where yield

increased by 97% by operating at 400-450 C. Small amount of dry air at ambient temperature can also be used to bring the temperature down.

5. The sulfur trioxide concentration at this stage is about 10% by volume. After cooling to 150 C by water and air heat exchanger and absorbed in oleum (fed at a rate to allow not over 1% rise in acid strength).

6. Final srcubbing is done with a lower strength acid (97%). Alternatively: 5. Hot sulfur trioxide passes through the heat exchanger and is dissolved in

concentrated H2SO4 in the absorption tower to form oleum: H2SO4(l) + SO3 (g) → H2S2O7(l)

6. Oleum is reacted with water to form concentrated H2SO4. H2S2O7(l) + H2O(l) → 2 H2SO4(l)

**Note that directly dissolving SO3 in water is impractical due to the highly exothermic nature of the reaction. Acidic vapor or mists are formed instead of a liquid.

H2SO4 PRODUCTION

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H2SO4 PRODUCTION

Major Engineering problems:

1. Design of multistage catalytic convertor for a highly exothermic reactions.

( 3 or more stages)

2. Optimization of space velocity in catalyst chamber i.e pumping cost vs fixed

charge of reactor.

3. Corrosion problem : Equilibrium yield can be increased by system pressure

but increases compression cost and corrosion dictates low pressure.

4. Adaptation of process to various gas feeds

5. Yield drops due to longitudinal mixing if gas velocity is too low.

6. Removal of heat of absorption of SO3 in acid. Pipe coolers with water

dripping over external surface have been replaced by cast iron pipe with

internal fins to promote better heat transfer.

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Contact process for making oleum and H2SO4

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Sulphuric acid Plant

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Acid Plant