The Contact Process

The manufacture of sulfuric Acid

Production of Sulfuric Acid

Sulfuric acid is made in several stages from SO2, obtained from the oxidation of sulphur or collection of SO2 from the smelting of sulfide ores such as copper, zinc or lead. This second collection of SO2 is very attractive as it is utilising the by-products of other processes and reduces emissions and waste.

SO2(g) SO3(g) H2SO4(aq)

In the following slides we will break down this process into three main steps.

1. Furnace or Burner (Only necessary if raw sulfur is used)

Air is cleaned by electrostatic precipitation, dried then heated to approx. 600oC.

Pure (liquid) sulphur is sprayed under pressure into the furnace, reacting with the oxygen in the air. The product is sulphur dioxide

S(l) + O2(g) SO2(g)

Alternative sources of sulphur dioxide are also used, either extracted from natural gas (some deposits contain a lot of hydrogen sulphide) or from the roasting of sulphide ores in the extraction of metals like zinc or lead. If so this stage can be skipped.

2. The converter

The converter contains trays or layers of porous pellets of a catalyst, vanadium (V) oxide (V2O5). The sulphur dioxide reacts with more air to form sulphur trioxide. This reaction is reversible and reaches an equilibrium. It is also an exothermic reaction and the temperature will rise to over 600oC. The mixture is continuously cooled to 400oC between each tray.

2SO2(g) + O2(g) 2SO3(g)

As the temperature rises the equilibrium shifts to the left (not forming SO3). To counter this the gases are allowed to cool slightly before they pass over the next layer of catalyst, by carefully controlling the process almost all sulphur dioxide is converted to sulphur trioxide

Yields and reaction rate

For the above reaction in the converter, the yield will increase as:

•The temperature _______________

•The pressure ____________

•And is excess reactant are added

But we have a compromise to make with reaction rates:

High reactions rates favour? ________

Thus conditions that are used as a compromise are:Moderate temperaturesModerate pressures (1 atm) – too expensive for high pressure!And use of a catalyst

2SO2(g) + O2(g) 2SO3(g)

3. The absorption tower

In practice the sulphur trioxide dissolves almost completely and is bubbled through concentrated sulfuric acid (that contains relatively little water) to form 98% sulfuric acid, known as Oleum (H2S2O7)

Sulphur trioxide will dissolve in water to form our final goal of sulfuric acid. However it is violently exothermic and usually results in a mist of sulfuric acid droplets that are very difficult to control.

a) SO3(g) + H2SO4(l) H2S2O7(l)

b) H2S2O7(l) + H2O(l) H2SO4(l)

Waste products

Most of the “waste” heat is recovered and used to heat water, in this way much of the energy can be reused. Because of this many sulfuric acid plants are co-located with other industrial processes.

Great care needs to be taken with the waste gases that are formed. There will be small amounts of sulphur dioxide, sulphur trioxide, sulfuric acid and possibly particle sulphur, all of which must be removed to prevent environmental damage. There is a double absorption method that can be used to prevent SO2 emissions. After a first round of processing through the converter, any SO2 that was not converted into SO3, can be collected and passed back through. SO2 that is released into the atmosphere can cause acid rain and respiratory irritants.

1 2

3

4

Dry air

Sulphur

SO2

sulfuric acid

Waste gases

SO3

Overview of the Contact Process

The Contact Process

Uses of Sulfuric Acid

The amount of sulfuric acid produced by a company is often an indicator of a countries industrial activity. Annual worldwide production is 170 million tonnes!

Transport and storage of sulfuric acid is hazardous, so most of the acid produced is used by alternate manufactures close to the production site.

Sulfuric acid is highly corrosive and burns skin and eyes.

For a large spill, the acid is treated with a natural hard substances such as clay or sand, then slowly diluted with water and finally neutralised with a base.

The main use of sulfuric acid in Australia is for fertiliser.

Uses

Other uses include paper, dyes, drugs and the acid is a main component of car batteries.

We utilise sulfuric made in Australia in a reaction with rock phosphate to make superphosphate (other fertilisers are ammonium nitrate and ammonium sulfate). This is a wonderful fertiliser for plant growth, as farm land often lacks phosphate required for crops. The finely powered rock phosphate is imported cheaply from north Africa and the reaction to make superphosphate takes a couple of weeks! WOW!

Sulfuric acid is also used as a strong acid, dehydrating agent and as an oxidant.

Let’s look at these uses a bit closer:

Uses

Sulfuric acid is diprotic. In a reaction with water, the first proton will be donated to from the hydronium ion and HSO4-. This reaction is virtually complete. The second reaction to form sulfate (SO4-) has a smaller Ka.

Before a sheet of iron is galvanised, we use sulfuric acid removes the iron(III) oxide layer.

WE ALWAYS ADD ACID TO WATER – AND VERY SLOWLY. THE REACTION IS VERY EXOTHERMIC AND EXCESSIVE HEAT IS GENERATED.

If we were to add water to acid, the small

amount of water boils instantly and cause

the acid to splatter everywhere!

Uses

Dehydrating agent – sulfuric acid dehydrates sugar into water and carbon, and also will dehydrate copper sulphate as shown below.

In the chemical industry , sulfuric acid is used to dry certain gas mixtures (such as N2 and CO2) for analysis. Ammonia gas is not able to be dehydrated by sulfuric acid as it is a base, and if mixed together it will react with the acid instead!

Sulfuric acid is also an oxidant, especially when hot!

Depending on the temperature and strength of the reactant, sulfur dioxide, sulfur and hydrogen sulfide gas can be produced by reaction with zinc and sulfuric acid. (see p.340)