DCDA Process for Sulphuric Acid Manufacture

The DCDA (Double Contact Double Absorption) process is a widely used method for the manufacture of sulphuric acid. In this process, sulphur dioxide gas (SO2) is oxidized to form sulphur trioxide (SO3), which is then absorbed in water to produce sulphuric acid. Let's break down the process into steps.

Oxidation of SO2

The first step of the DCDA process is the oxidation of SO2. This step is carried out in a reactor called the "converter". The converter is a large vessel where SO2 gas is mixed with air and passed over a catalyst bed. The catalyst used is usually vanadium pentoxide (V2O5), which helps to promote the reaction.

The reaction is exothermic, meaning that heat is released during the process. The temperature in the converter is maintained at around 450-500°C to ensure that the reaction proceeds efficiently. The oxidation of SO2 produces SO3 gas.

SO2 + 1/2 O2 → SO3 + heat

Absorption of SO3

The next step is the absorption of SO3 gas in water to produce sulphuric acid. This step is carried out in two stages: the first absorption stage and the second absorption stage. The purpose of having two stages is to improve the efficiency of the process and increase the concentration of the acid produced.

In the first absorption stage, SO3 gas is mixed with a small amount of water to form a mist. The mist is then passed through a packed tower called the "intermediate absorber". The intermediate absorber contains a packing material that provides a large surface area for the acid to come into contact with the mist. The acid that is produced in this stage is called "intermediate acid" and has a concentration of around 65-70%.

In the second absorption stage, the intermediate acid is mixed with more water and passed through a second packed tower called the "final absorber". The final absorber operates at a lower temperature and higher pressure than the intermediate absorber. This helps to increase the concentration of the acid produced. The acid that is produced in this stage is called "final acid" and has a concentration of around 98%.

Recovery of Heat

The DCDA process is highly exothermic, meaning that a large amount of heat is released during the process. This heat is recovered and used to generate steam, which can be used for various purposes in the plant. The heat recovery system consists of a series of heat exchangers and a steam turbine.

Conclusion

In conclusion, the DCDA process is a highly efficient method for the manufacture of sulphuric acid. The process involves the oxidation of SO2 to form SO3, which is then absorbed in water to produce sulphuric acid. The process is highly exothermic, and the heat released during the process is recovered and used to generate steam.