Sulfuric acid production


Sulfuric acid is used in a wide range of processes, it is used in metal treatment, for drug manufacture, for paint, and the making of explosives. Sulfuric acid is formed by burning sulfur in a furnace to form sulfur dioxide. In a converter the sulfur dioxide reacts
with more air to form sulfur trioxide. In absorbers, the sulfur trioxide mixes with fairly
concentrated sulfuric acid to increase the concentration of the
acid. The acid is then diluted to the concentration needed by the customer. Some acid is recycled through the
absorbers. Some is stored for sale. At this small plant, that make 600 tonnes acid per day, the sulfur is burnt in a furnace. Sulfur dioxide passes to the converter. sulfur trioxide is fed to the
absorption towers to produce acid of the required
concentration. This is then stored for delivery by tanker. This tanker is delivering sulfur. It’s obtained from local refineries
where it’s been recovered from oil and gas. It’s transported as a liquid at around 140
degrees Celsius. The liquid sulfur is sampled before
being stored for acid production. Air the second raw material is drawn from the atmosphere, dried and compressed. The sulfur is sprayed into a furnace, where it burns in
the dry air. Sulfur combines with oxygen in the air to
form sulfur dioxide. The oxidation produces large amounts of heat. That heat is used to boil water, to produce steam, to generate electricity. The gas mixture which leaves the furnace contains sulfur
dioxide and enough oxygen for the next stage. In the converter, sulfur dioxide reacts with oxygen to
produce sulfur trioxide. The reaction requires a catalyst mainly a vanadium(V) oxide. It is most effective at 440 degrees Celsius. There are four beds of catalyst in the converter. The first catalyst bed converts about 63% of the sulfur dioxide to sulfur trioxide, and heat is produced. The gas stream is cooled before it passes to the next catalyst bed. Hot gas super heats steam from the furnace boiler. The gas stream passes through two further catalyst beds and the percentage conversion increases. Between each stage the hot gases have to be cooled. By the third bed, about 95% of the original sulfur
dioxide has been converted to sulfur trioxide. After the third catalyst bed, the gas stream is led in to the bottom of an absorption tower packed with ceramic material. The rising gas meets a stream of hot sulfuric acid that runs down the absorber. The sulfur trioxide reacts with the water in the acid to increase the concentration of the
acid to about 99%. The gas from the top of the tower still contains some sulfur dioxide and
oxygen, and is fed back into the final bed of the converter after which 99.5% of the original sulfur dioxide has changed to sulfur trioxide. The 99% concentrated acid is diluted with water to about 98%, for example, and this increases the volume of the acid. This extra 1% or so is the volume of the acid that has been
made. The acid concentration is checked by sampling. Beside sulfuric acid, the plant generates a large amount of heat which is used to produce high temperature steam and make electricity – electricity is sold to
the national grid. The production process is controlled and monitored from a control room. Acid is transported to customers by rail and by road.

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