Production of Sulfuric Acid

Chapter 21

Key Knowledge

The industrial production of sulfuric acid Factors affecting the production,

including rate and equilibrium position, catalysts, temperature, pressure

Waste management including generation, treatment and waste reduction

Health and safety Uses of Sulfuric acid

Chapter OutcomesName the raw materials used in the

production of sulfuric acidDescribe the reaction steps by which

sulfuric acid is manufacturedExplain how the principles of

equilibrium and reaction rates play a significant role in determing reaction conditions

Explain the reasons for choice of reaction conditions, such as pressure, temprrature and catalysts.

Chapter Outcomes

Describe waste management procedures

Describe health and safety issues involved in the production of sulfuric acid

Describe how the production of sulfuric acid through the contact process realates to the principles of green chemistry

Recall the major uses of sulfuric acid

Key terms

Absorption towerContact processConverterDehydrating agentDiprotic acidDouble absorptionoleum

Sulfuric Acid

Sulfuric acid is produced in greater quantities than any other chemical in both Australia and the world.

Annual worldwide production is estimated at about 170 million tonnes and Australian production at 4 million tonnes.

In future years it is anticipated that Australia will become a major exporter of the chemical.

Sulfuric Acid

Transport and storage of sulfuric acid are hazardous.

A high proportion of the acid is used close to the site of manufacture.

Most sulfuric acid plants are located near smelting and refining industries that produce waste sulfur dioxide, a raw material for the production of sulfuric acid.

Uses of sulfuric acid It is also used in

the manufacturing of paper, household detergents, pigments, dyes and drugs.

It is the electrolyte in car batteries.

Superphosphate

Many Australian soils are phosphorous deficient and it must be added to the land.

During superphosphate manufacture, insoluble calcium phosphate contained in rock phosphate is converted to a soluble form that plants can absorb.

This reaction takes several weeks to occur:

Ca3(PO4)2(s) + 2H2SO4(l) + 4H2O(l) → Ca(H2PO4)2(s) + 2CaSO4.H2O(l)

superphosphate

Superphosphate

The final mixture is superphosphate. It is crushed into a powder and bagged for easy distribution.

Although Queensland has deposits of phosphate rock, it is not really used as a reactant.

Instead we use rock phosphate that comes from North Africa as it is cheap and readily available.

Used as a strong acid

Pure sulfuric acid is a viscous liquid that reacts with water in two steps.

H2SO4(l) + H2O(l) → HSO4 - (aq) + H3O +(aq) Ka

= 109 mol L-1

HSO4 - (aq) + H2O(l) <==> SO4

2- (aq) + H3O +

(aq)

Used as a strong acid

Sulfuric acid is a diprotic acid. The first step proceeds virtually to

completion. The second step has a much smaller

Ka value.H2SO4(l) + H2O(l) → HSO4

- (aq) + H3O +(aq) Ka = 109 mol L-1

HSO4 - (aq) + H2O(l) <==> SO4

2- (aq) + H3O +(aq) Ka = 1.0 x 10-2 mol L-1

It is used as a strong acid in the ‘pickling’ of iron and steel. This is where the iron(III) oxide is removed from the surface of the iron.

Strong Acid

A large amount of heat is evolved during this process.

For this reason when preparing sulfuric acid, you ALWAYS add the acid to water slowly with continuous stirring.

Never add water to acid as this can cause the water to boil and the acid to splatter.

As a dehydrating agent Concentrated sulfuric acid is a powerful

dehydrating agent. Sugar is dehydrated:

C12H22O11(s) 12C(s) + 11H2O(l)

The dehydrating ability of sulfuric acid is often utilised in laboratories to dry gas mixtures that are being prepared or analysed.

It is not suitable for bases as they will react with the acid

H2SO4(l)

As an oxidant

Concentrated sulfuric acid is a strong oxidant, especially when hot.

Sulfuric acid can be reduced to sulfur dioxide (SO2), sulfur (S) or hydrogen sulfide (H2S), depending on the temperature, the strength of the reductant involved and the mole ratio of the reactants.

As an oxidant

The following reactions can occur when zinc is added to sulfuric acid:

Zn(s) + 2H2SO4(aq) → ZnSO4(aq) + 2H2O(l) + SO2(g)

3Zn(s) + 4H2SO4(aq) → 3ZnSO4(aq) + 4H2O(l) + S(s)

4Zn(s) + 5H2SO4(aq) → 4ZnSO4(aq) + 4H2O(l) + H2S(g)

As an oxidant

Like other strong acids, dilute sulfuric acid reacts with zinc to produce hydrogen gas:

Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)

What is the oxidant?? H+ (g)

Manufacturing Sulfuric Acid:The contact processSulfuric Acid is manufactured in

stages from sulfur dioxide.These involve oxidation of sulfur

dioxide to sulfur trioxide.Followed by conversion to the acid.The process can be summarised:

SO2(from various sources) → SO3 → H2SO4

The contact process

The contact process – raw materials The sulfur dioxide used to produce sulfuric

acid is obtained from two principal sources Combustion of sulfur recovered from natural

gas and crude oil Sulfur dioxide formed during the smelting of

sulfide ores of copper, zinc or lead. A third process can be used from mining of the

underground deposits of elemental sulfur but this is not used in Australia due to the first two being in high abundance.

Step 1: Burning Sulfur

If sulfur is used as a raw material, the first step is to spray molten sulfur under pressure into a furnace up to 1000°C.

Here it burns in air to produce sulfur dioxide gas. The sulfur dioxide gas is then cooled for the next step

The high surface area of the sulfur spray allows combustion to be rapid.S(l) + O2(g) → SO2(g); ∆H = -297 kJ mol-1

Step 2: Catalytic oxidation of sulfur dioxide Sulfur dioxide gas is oxidised to sulfur

trioxide gas by oxygen, using Vanadium oxide as a catalyst.

2SO2(g) + O2(g) 2SO3(g); ∆H = -197 kJ mol-1

This step is performed in a reaction vessel called a converter.

Sulfur dioxide is mixed with air and passed through trays containing loosely packed porous pellets of catalysts.

Step 2: Catalytic oxidation of sulfur dioxide The converter contains several catalyst beds and

the gas mixture passes over each in succession. Because the reaction is exothermic it is

necessary to cool the gas mixture as it passes from one tray to another to maintain the desired reaction temperature.

The temperature in the converter is maintained between 400°C and 500°C and the pressure is close to 1 atm.

Nearly complete conversion of sulfur dioxide to sulfur trioxide is achieved.

Stage 2: Equilibrium yieldUsing Le Chatelier’s principal, the

equilibrium yield of sulfur trioxide will increase: As temperature decrease. Since the

reaction is exothermic a decrease in temperature will favour the forward reaction.

As pressure increases. Since there are more gas particles on the reactants the forward reaction will result in a decreased pressure.

If excess reactants are added.

Stage 2: Rate of reaction

The rate of reaction will be faster: As temperature increases As pressure increases If a catalyst is employed

What compromises have been made to get the fastest reaction with the best yields?

Step 3: Absorption of sulfur trioxideSulfur trioxide reacts with water to

form sulfuric acid:

SO3(g) + H2O(l) → H2SO4(aq); ∆H = -130 kJ mol-1

However direct reaction with water is not used, because so much heat evolves when sulfur trioxide is added to water that a fine mist of acid is produced which is difficult to collect.

Stage 3: Absorption of sulfur trioxide Instead, sulfur trioxide gas is passed into

concentrated sulfuric acid in an absorption tower. This reaction occurs in two steps

1. The sulfur trioxide gas dissolves almost totally in the acid to form a liquid known as oleumSO3(g) + H2SO4(l) → H2S2O7(l)

2. Oleum obtained from the absorption tower is then carefully mixed with water to produce sulfuric acid: H2S2O7(l) + H2O(l) → H2SO4(l)

Summary of contact process1. Oxidation of S to SO22. Catalytic oxidation of SO2 to SO33. The absorption of the SO3 by

previously prepared sulfuric acid to produce oleum, H2S2O7

4. The dilution of the oleum with water to make sulfuric acid.

Waste Management

Sulfuric acid plants use sulfur or sulfur dioxide that is a by-product from other industries.

To maximise their conversion of sulfur dioxide to sulfur trioxide most plants now use a double absorption process.

Any unreacted gas from the absorption tower is passed over the catalytic beds again and re passed through the absorption tower.

This improves the percentage of sulfur dioxide converted from 98% to better than 99.6%

Waste Management

Emissions from the plant have to be continuously monitored for sulfur dioxide as this can cause acid rain.

The amount of sulfuric acid mist emitted from the process is minimised by controlling the operating temperature of the absorber, gas flow rates and concentrations.

Waste Management

Improvements in conversion have also been made by adding small amounts of caesium to the vanadium oxide catalyst to increase its efficiency and allow it to operate at lower temperatures

Caesium-doped catalysts are about 3x more expensive than the usual vanadium oxide catalyst.

Waste Management

There is relatively little solid waste produced from sulfuric acid manufacturing.

The catalyst is dumped in landfill after recovering the mildly toxic vanadium.

The cooling water is recycled.All three processes are exothermic,

meaning energy is produced. This energy is used to generate its electricity or as a source to produce other chemicals.

Health and Safety

Sulfuric acid is highly corrosive and can burn skin and eyes severely.

It can cause blindness and third degree burns on contact.

Exposure to sulfuric acid mist can cause other health problems.

Workers in sulfuric acid plants can also be exposed to the acid through breathing air contaminated with emissions containing oxides of sulfur

Health and Safety

Strict safety procedures including adequate methods to trap the fumes are required to minimise the risks to workers and the environment in the case of accidental release

Work areas must be well ventilated and employees wear protective clothing.

Acid spills are contained using materials such as earth, clay or sand and then slowly diluted with water before being neutralised with a base such as limestone or sodium carbonate