How is the Flue Gas Scrubbed in a Double Loop Scrubber?

This article describes how the flue gas moves through a double loop scrubber and what happens to the gas as it moves through the scrubber.  Some of the functions of the scrubber internals are also described.  


Figure 1 shows a typical double loop tower.

Figure 1

Double loop scrubbers are called double loops because there are two pumping circuits.  One pumping circuit is the quencher recycle loop and the other pumping circuit is the absorber recycle loop.  Figure 2 shows a typical quencher recycle loop and absorber pumping loop. 

The quencher recycle pump pumps slurry from the quencher to the quencher sprays. The slurry from the quencher sprays fall back into the quencher.

The absorber recycle pump pumps slurry form the absorber feed tank up to the top sprays above and below the tray.  The slurry from the top sprays are collected into the bowl and returned to the absorber feed tank by the bowl return line; hence, this makes up the complete pumping circuit for the top loop.

Figure 2

The Quencher Section

Referring to figure 3, the flue gas enters the tower and encounters the quencher section of the tower where the quencher recycle pump sprays slurry into the incoming gas counter-currently.  The reason for spraying the incoming gas is to cool the gas from 300 F to 130 F.  The flue gas enters the tower tangentially (see figure 4) where the gas swirls in the quencher to minimize carry up of the slurry into the bowl section of the tower.  The swirling of the gas also decreases the potential to carry up chlorides into the absorber feed tank.  The quencher section chloride concentration is normally much higher than the absorber feed tank chloride concentration; hence, the quencher is usually made of alloys such as Inconel that resist corrosion.  The absorber feed tank is usually made up of carbon steel or fiber glass.  Sulfur dioxide is minimally removed in the quencher section.  Two to five percent of the sulfur dioxide is removed in the quencher section.

Figure 3

Figure 4

The Bowl and Tray Section

Referring to figure 5, once the quenched flue gas leaves the quencher section, the swirling of the gas is stopped by the vortex breaker which is the bottom part of the bowl section.  The gas profile goes from swirling to a straighter flow around the bowl.  At this point, the flue gas enters the absorber section where it encounters the absorber recycle loop.  The absorber recycle loop sprays slurry into the gas stream counter-currently.  The source of the slurry comes from the absorber feed tank (AFT).  The gas enters a section of spray that is below the tray, in this zone some removal takes place. Then the gas moves through the sieve tray where it encounters slurry on the top of the tray.  The slurry on top of the tray has a liquid level where the gas moves through.  The liquid to gas contact and residence time gives the highest removal of sulfur dioxide in this section.  Once the gas leaves the tray section, it encounters the sprays above the tray where some more sulfur dioxide is removed.  The slurry level that is formed on the tray comes from sprays above the tray.

Figure 5

Mist Eliminator Section

Finally, the gas enters the mist eliminator section sometimes called demisters (See figure 6).  The mist eliminators remove any slurry droplets entrained into the gas stream as the gas moves through the tower.  The mist eliminators are vanes that physically traps the slurry droplets. Once the droplets adhere to the vanes, the vanes are then washed with water through the mist eliminators sprays to remove the slurry from the vanes.  The larger droplets then fall back into the tower.

After leaving, the mist eliminator section, the cleaned flue gas makes its way through the outlet duct and then through the stack.  The removal efficiency of this type of scrubber can be 90% to 98% depending on the scrubber chemistry and the liquid to gas ratio.

Figure 6