The Contact Process

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The Contact Process

• The manufacture of sulfuric Acid

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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.
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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.
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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
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Yields and reaction rate
2SO2(g) O2(g) ? 2SO3(g)

• 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 temperatures Moderate pressures (1 atm)
too expensive for high pressure! And use of a
catalyst
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3. The absorption tower
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.
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)

1. SO3(g) H2SO4(l) ? H2S2O7(l)
2. H2S2O7(l) H2O(l) ? H2SO4(l)

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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.
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Overview of the Contact Process
Dry air
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sulfuric acid
SO2
1
SO3
2
4
Waste gases
Sulphur
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The Contact Process
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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.
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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. Lets look at these uses a bit closer
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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!
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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)