Acid plant tail gas characteristics

Acid plant tail gas contains SO2, SO3, H2S04( ) (acid mist), NO O2, N2, and sometimes CO2. It is dry, typically containing less than 50 mg/Nm H2O or 60 ppmv. 

Tail gas SO2 concentration varies with the acid plant configuration, single contact or double contact, and with the feed gas SO2 concentration as shown in Fig. 28.1. 

Single contact acid plants oxidize 98-99% of their feed gas SO2, whereas double contact acid plants oxidize 99-99.7% with four catalyst beds and little or no cesium-promoted catalyst. The highest SO2 oxidation efficiencies, at around 99.95%, are 

The only way to lower tail gas SO2 concentrations below those in Fig. 28.1 is by sending the gas through a scrubber. Tail gas scrubbers are often used any time the acid plant is online however, some are used only as start-up scrubbers meaning that they are only used when the acid plant is being brought online under less-than-optimum SO2 oxidation conditions in the catalyst beds. These conditions typically only last for a few hours and would result in SO2 emissions greater than permitted levels without the tail gas scrubber operating. 

Acid mist is always present in acid plant tail gas in the range of 30-50 mg/Nm as pure H2SO4. When the feed gas passes through the drying and absorption towers, droplets of acid are carried with the gas. These droplets are usually small in the range of 1-3 pm diameter. Most, but not all, of this acid is removed in the mist eliminators located in the top of the towers (Fig. 9.3). 

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