Candles, acid mist removal

Fig. 9.3. Acid mist removal candle filter being installed atop a stainless steel H2SO4 making tower. It is one of many. Exiting gas passes inward through the candle fabric and out the top of the candle - then out of the tower. The acid mist is caught in the candle fabric by impact, diffusion and Brownian forces (Brink, 2005 Friedman and Friedman, 2004 Lee and Byszewski, 2005 Ziebold and Azwell, 2005). The large total area of the candles gives a low gas velocity through the fabric, which allows 99+% capture of the mist. The captured mist trickles down the fabric and drips back into the tower or into collection pipes (Outokumpu 2005).

Figure 9.16 shows a typical flow arrangement. The fibers trap the acid mist and the liquid then trickles to the bottom of each candle. There, it must be collected and brought to a drain for removal. A liquid seal on the acid drain prevents bypassing of the chlorine gas. The tail pipes, usually of CPVC, can be sealed by maintaining a level of acid in the bottom of the vessel or each element can be sealed separately with an overflow cup attached to the bottom of the drain pipe. In the latter case, there still must... 

The mist eliminator can be internally or externally mounted. The filtering elements will be suspended candles, and the acid they capture will drain into the acid receiver. Section 9.1.5 treated the subject of removal of acid mists. 

Porous candles used only for small flow-rates Pads made from glass fibre—removal of acid mist Pads made from stainless steel—/or removal of acid mist Layers of crushed fire bricks—/or removal of ash particles Crushed coke piece (temp, not above 200 °C)... 

Acid mist captured on the candle filters drains by gravity through collection pipes to the top of the packed bed or to a collection tank external to the absorption tower. These candle drips are sometimes treated to remove nitrate compounds that form when nitrogen oxides (mainly NO(g) and N02(g)) in the acid plant feed gas react with sulfuric acid to form nitrosyl sulfuric acid (HN0S04(f) Daum, 2000 Lyne et al., 2002). High nitrogen oxide concentration feed gas is commonly found in metallurgical acid plants. 

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