Final absorption tower

Downstream from the 3rd bed, the gas is cooled and passed to an intermediate absorption tower, in which the S03 formed is absorbed in recirculating sulphuric acid. The cold and practically S03-free process gas is reheated to 380-440°C and returned to the converter, where the remaining SO2 is converted to S03 in a 4th catalyst bed. The rest of the S03 is subsequently recovered in a final absorption tower before the process gas, containing a small fraction of unconverted S02, is emitted through the stack. The combustion air is dried with the 98 wt% product acid in order to avoid corrosion and acid mist problems in the plant. The sulphuric acid process normally operates close to atmospheric pressure with the combustion air blower dimensioned just for compensation of the pressure drop through the plant. 

In the process (Fig. 1), sulfur and oxygen are converted to sulfur dioxide at 1000°C and then cooled to 420°C. The sulfur dioxide and oxygen enter the converter, which contains a catalyst such as vanadium pentoxide (V205). About 60 to 65% of the sulfur dioxide is converted by an exothermic reaction to sulfur trioxide in the first layer with a 2 to 4-second contact time. The gas leaves the converter at 600°C and is cooled to 400°C before it enters the second layer of catalyst. After the third layer, about 95% of the sulfur dioxide is converted into sulfur trioxide. The mixture is then fed to the initial absorption tower, where the sulfur trioxide is hydrated to sulfuric acid after which the gas mixture is reheated to 420°C and enters the fourth layer of catalyst that gives overall a 99.7% conversion of sulfur dioxide to sulfur trioxide. It is cooled and then fed to the final absorption tower and hydrated to sulfuric acid. The final sulfuric acid concentration is 98 to 99% (1 to 2% water). A small amount of this acid is recycled by adding some water and recirculating into the towers to pick up more sulfur trioxide. 

In a variant to the above process, provision is made by means of a glass constmc-tion/MS-PTFE absorption tower (with glass internals—tower packing) parallel to the final absorption tower along with the glass acid cooling system. 

H2O2 is therefore used for oxidizing the remaining traces of (dissolved) SO2 in the circulating acid in the plant. This minimizes the escape of SO2 from tail gases from the final absorption tower. 

A separate acid circuit will be provided for the final absorption tower to minimize SO2 content in the stack. 

Droplets of H2SO4 resulting from absorption. H2SO4 mist emission due to inefficient absorption and inefficient demister provided in the final absorption tower. Modem plants also install specially designed candle type demisters in the FAT exit. Many other pollutants may be emitted in trace amounts depending on the source... 

Emissions from S.A. plants generally consist of some unconverted SO2, unabsorbed SO3, and acid mist escaping from demisters installed in final absorption tower. These are minimized by the following techniques ... 

Sulfur dioxide percentage in gases at exit of final absorption tower... 

Separate acid circulation circuit for final absorption tower to avoid stripping of dissolved SO2 from the inter-pass absorption tower acid circuit which can occur if a common acid circulation tank is used. 

If snccessful, this will be an innovative and impactful contribution that avoids the conventional converter, drying towers, interpass and final absorption towers, heat exchangers, add coolers, etc. 

Apart from the benefits of the high pressure mixing of SO3 with water to produce sulfuric acid, the proposed cold process for the manufacture of sulfuric acid has also been conceived to avoid the complexity of requiring a sulfur furnace and the related heat recovery system, the multipass static converter, counter current heat exchangers, the interpass absorption tower (IPAT), drying tower (DT), final absorption tower (FAT), mist eliminators, acid coolers, and alkali scrubber. The resulting plant is, as a result, of much lower cost in equipment and land use. 

The following details should be separately recorded for each of the acid towers (drying tower, inter pass absorption tower, final absorption tower)... 

The pumps for acid circulation are most important in the acid plant. It can also be said that this pump is the heart of the plant and hence any malfunction or failure in operation results in stopping the plant and hence is not acceptable. These pumps are used to circulate acid through the intermediate absorption tower, final absorption tower, and drying tower. 

Minimise the escape of unreacted inputs from the plant by better process. A good example is the modified 3 + 2 DCDA process for the production of sulphuric acid wherein five catalyst beds are used instead of four. Three beds are used before the interpass absorption tower and two are after it. This results in the conversion of up to 99.85 % of SO2 to SO3. By providing separate acid circuit for the final absorption tower, the emission of SO2 in exit gases can also be brought down further as compared to the earlier design where aU acid towers had a common circulation tank. 

An interconnection of plant units with final absorption tower pump and scrubbing liquor pump connection to alarm/hooter) in a sulphuric acid plant. 

The air blower will trip if the acid circulation pump for final absorption tower trips. 

Absorption/distillation towers shall have packings with glazing/non-corrosive material to prevent contamination of process streams which shall be tested for compatibility with process fluids at operating conditions (e.g. the tower packing in any of the drying, interpass absorption or final absorption towers of the sulphuric acid plant shall not get affected by 98.5 % sulphuric acid at 120 °C). 

Final absorption tower Absorption of SO3 Environmental pollution Provide alarm if exit gases pollute the environment... 

ImpropCT conversion of SO2 in converter Environmental pollution due to unconvMted SO2 Run alkali scmbber after final absorption tower continuously Online moniuning of SO2 in exit gases with alarm... 

Following the third catalyst pass, gas is treated in the interpass absorption tower to absorb SO3. Exit gas from the interpass tower is reheated by heat exchange and passes to the fourth catalyst bed for conversion of residual SO2, and then to the final absorption tower. With this double absorption system, SO2 conversion can exceed 99.5 per cent. 

Product acid is drawn from fhe final absorption tower circuit and is cooled to 40°C before storage in mild steel tanks. 

Treatment of Sulphuric Acid Plant Tail Gas from Final Absorption Tower... 

The boiler feed water is condensed steam after electricity generation that has been conditioned in a deaerator. The boiler feed water is preheated in economizers prior to entering the heat recovery boiler. Economizers are heat exchangers that transfer heat to boiler feed water while cooling gases entering intermediate and final absorption towers (Chapter 9). 

Table 9.1 Comparison of Fig. 9.6 s intermediate and final absorption towers ...

The top-packed bed is fed with cool ( 350 K) acid from the acid plant s final absorption tower (Fig. 9.6). Its principal purpose is to absorb SO3, H20(g), and H2S04(g), rising from the hot bottom bed. The vapor pressures of all these gases are all lower with cool acid than with hot acid. 

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