Heavy Chemical Industry

HEAVY CHEMICALS
Heavy Chemicals
Heavy chemicals are those chemicals whose
consumption as well as production are in
large (heavy) amounts as compared to other
chemicals.
Heavy chemicals includes:
Sulphuric acid
Ammonia
Nitric acid
Hydrochloric acid
Sodium hydroxide etc.
SULPHURIC ACID
Sulphuric acid (H2SO4)
It is often called king of chemicals as it is most widely used and
most important technical product.
It is an agent for sulphonation and sulphate formation but most
frequently it is used because it is a rather strong and most cheaply
priced inorganic acid.
It is a dibasic acid and acts as a dehydrating and oxidizing agent.
Sulphuric acid forms different hydrates each having different
melting point and each solution have different specific gravities.
Sulphuric acid is also sold in the form of solutions of SO 3 in H2SO4.
This is called oleum (H2S2O7)
20% oleum means that in 100 kg there are 20 kg of SO 3 and 80 kg
of H2SO4.
Hydrates Of Sulphuric acid
Sulphuric acid fuming H2S2O7
100 % acid H2SO4
Monohydrate H2SO4. H2O
Dihydrate H2SO4.2H2O
Previously when much of acid was made by chamber process,
sulphuric acid solution in water was sold according to their
specific gravities or their degree baume (°Be’). Reference
temperature for specific gravity and degree baume is 15°C.The
specific gravity increases gradually to 1.844 at 15 °C for 97%
acid after which it decreases to 1.839 at 15 °C for 100% acid. As
contact process has replaced chamber process the sulphuric acid
percentages are normally specified.
USES Of H2SO4
Fertilizer manufacture is the greatest use of sulphuric acid . e.g, SSP etc.
SSP and TSP plants are always near the sulphuric acid plants to minimize
transportation cost.
In the manufacture of sulphates of ammonia, copper, aluminum, zinc, iron
etc.
In the manufacture of organic acids such as citric acid, oxalic acid etc.
In producing heavy metals and in electroplating.
In the purification of petroleum products.
Oleum is needed in nitrocellulose, nitroglycerine, TNT and dye manufacture.
In the synthesis of phenol, alkylation of isobutane, and in recovery of fatty
acids in soap manufacture.
In the pickling of iron and steel before galvanizing and tinning.
Manufacture of sulphuric acid
Two methods:
Lead chamber process
Contact process
Lead chamber process is obsolete now because
of lead used as a catalyst is highly expensive
and pollution causing.
One of the disadvantage which led to demise of
chamber process is that it can produce acid of
only 78% strength. More energy is required in
this process.
Manufacture By the Contact Process
This process involves the passing of a mixture
of sulphur dioxide and air over a catalyst and
then the absorption of the sulphur trioxide in
98.5-99% sulphuric acid.
Initially platinum is used as a catalyst but
with the passage of time vanadium (V2O5)
catalyst came into being and completely
replaced the platinum catalyst.
The reactions involved are:
S(l) + O2 (g) SO2(g) ∆H=-298.3 kJ, 25 °C
SO2(g) + ½ O2(g) SO3(g) ∆H=-98.3 kJ, 25 °C
SO3(g) + H2O (l) H2SO4(l) ∆H=-130.4 kJ, 25 °C
Two types of configuration are there:
Single absorption configuration
Double absorption configuration
In single absorption less attention was given to
efficient heat recovery so that all heat produced
during recovery and conversion is utilized in
producing steam.
The contact process has been gradually modified to
use double absorption process which increases yield
and reduces stack emission of unconverted SO2.
Conversion using the single absorption is typically about
97-98%.
In double absorption, the gases leaving the first
absorption tower are reheated by the heat exchanger with
the bottom converter gases and reenter the final stage of
the converter. So the reaction of sulphur trioxide proceed
more in the desired direction and higher recovery 99.5%
can be achieved. The gases leaving this final stage are
cooled and SO3 are absorbed in the final absorber tower.
The heat of combustion of sulphur is utilized in a waste
heat boiler and economizers to generate steam for melting
the sulphur and for power purposes around the plant.
The reaction from SO2 to SO3 is an exothermic reversible
reaction. The equilibrium conversion data show that the
conversion of sulphur dioxide to sulphur trioxide decreases
with the increase in temperature. For this reason it is
desired to carry out the reaction at low temperature. The
reaction takes place by allowing the gases to enter over
the catalyst at about 425-440° C.
An increase in pressure will favor the reaction to proceed
in the forward direction according to the Le Chatlier’s
principle. But the effect is not so large.
If a portion of SO3 is removed the reaction will move in
the forward direction. This effect is used in the process to
get maximum conversion efficiency.
In double absorption configuration, the gases leaving the
converter after having passed through two or three layers
of catalyst are cooled and passed through an intermediate
absorber tower where some of the SO3 is removed. Mist
eliminators are essential when inter pass cooling is used.
The gases leaving the tower are then reheated and they
flow through the remaining layers of catalysts in the
converter. The gases are then cooled and passed through
the final absorber tower before being permitted to flow
into the atmosphere.
In this manner more than 99.7% of total SO2 is converted
into SO3 and subsequently into sulphuric acid.
Assignment 4
Prepare a report on ‘heavy chemicals’,
report must include:
Introduction, applications and
manufacturing processes along with
flowsheets of the heavy chemicals
being produced in Pakistan.

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HEAVY CHEMICALS

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