Tail Gas Scrubber

Make sure the circulation pumps and pH controller are always in working order. There should always be adequate stocks of 50% caustic solution available. 

Gas inlet/outlet valves must be in working order. 

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Toxic pollutants found in the mercury cell wastewater stream include mercury and some heavy metals like chromium and others stated in Table 22.8, some of them are corrosion products of reactions between chlorine and the plant materials of construction. Virtually, most of these pollutants are generally removed by sulfide precipitation followed by settling or filtration. Prior to treatment, sodium hydrosulfide is used to precipitate mercury sulfide, which is removed through filtration process in the wastewater stream. The tail gas scrubber water is often recycled as brine make-up water. Reduction, adsorption on activated carbon, ion exchange, and some chemical treatments are some of the processes employed in the treatment of wastewater in this cell. Sodium salts such as sodium bisulfite, sodium hydrosulfite, sodium sulfide, and sodium borohydride are also employed in the treatment of the wastewater in this cell28 (Figure 22.5). 

In the eady 1970s, air pollution requirements led to the adoption of the double contact or double absorption process, which provides overall conversions of better than 99.7%. The double absorption process employs the principle of intermediate removal of the reaction product, ie, S03, to obtain favorable equilibria and kinetics in later stages of the reaction. A few single absorption plants are still being built in some areas of the wodd, or where special circumstances exist, but most industtialized nations have emission standards that cannot be achieved without utilizing double absorption or tail-gas scrubbers. A discussion of sulfuric acid plant air emissions, control measures, and emissions calculations can be found in Reference 98. 

Figure 3.22 A schematic of the Haldor-Topsoe A/S Formaldehyde Process. (I) pump, (2) heat exchanger, (3) reactor, (4) low pressure steam boiler, (5) absorber, (6,7) cooling cycles, (8) tail gas scrubber, (9) reactor heater, (10) methanol evaporator, (11) boiler feed-water preheater.

Caesium-promoted catalyst for sulphuric acid plant has a lower ignition temperature and can result in higher overall conversion of SO2 SO3 but is more costly. It can reduce consumption of alkali in tail gas scrubber as compared to conventional potassium-promoted catalyst. 

Forced draught fans and tail gas scrubbers (if the gases contain H2S or other sulphurous compounds) are to be provided as per need. The gas supply shall be automatically cut off if the air supply pressure to furnace gets reduced below set point due to any reason. This is to prevent explosions. 

Less flow of scrubbing liquor/low level in tail gas scrubber tank Environmental pollution may occur Low flow alarm test level indicator alarm every week Provide automatic addition of alkali to scmbbing liquor tank... 

Escape of CI2 or HCl from tail gas scrubber Can be dangerous Provide alkali scrubbers, exit gas analysers, and gas detectors in working areas Provide installed spare circulation pumps for scmbber install gas detectors for CI2 and HCl and alarms... 

Tail gas scrubbers for phosphate operations typically utilize very dilute aqueous solutions. Data on the vapor pressure of HF and S1F4 over solutions containing less than 2.S percent H2S1F6 are given in F [ures 6-14 and 6-15 which are based on Russian data (Illanionev, 1963) as presented by Hansen and Danos (1980). 

In this installation, the jet scrubber units are constructed of fiber reinforced plastic (FRP) with PVC spinners in the spray nozzles. The tail-gas scrubber towers, tank, and ductwork are also constructed of FRP with polypropylene packing and mist eliminators. 

Hay, S., Porretta, F., Wiggins, B., 2003. Design and start-up of acid plant tail gas scrubber. In Diaz, C., Kapusta, J., Newman, C. (Eds.), Copper 2003-Cobre 2003 Volume IV-Pyrometallurgy of Copper (Book 1). The Herman Schwarze Symposium on Copper Pyro-metaUurgy. CIM, Montreal, (Juebec, pp. 555 566. 

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. 

Seawater scrubbers make use of the natural alkalinity in seawater (sodium bicarbonate, NaHCOs). Seawater, at a pH of 7-8.2, is pumped to the tail gas scrubber. SO2 is absorbed and reacted in the solution which is then returned to the sea at a pH above 6.0 (Sagedahl, 2007). 

Details of several operating industrial acid plant tail gas scrubbers are shown in Tables 28.3 and 28.4. 

Table 28.3 Details of two industrial acid plant tail gas scrubbers—Cont d ...

Operating costs are almost always directly related to the chosen reagent. Operating costs for acid plant tail gas scrubbers usually range from US 50 to 400/tonne SO2 removed. Reagent unit costs generally increase as follows (from top to bottom) ... 

Tail gas scrubbers often use basic solutions to further remove sulfur compounds occurring as SO2, SO3, and H2SO4 mist. Most are operated continuously and some only during start-up conditions. They are very efficient, often lowering tail gas SO2 concentrations from 200-3000 ppmv down to <20 ppmv. 

Tail gas scrubbers are used to treat single contact and double contact acid plant tail gas. The choice of technology is based on a combination of required SO2 removal efficiency along with reagent costs and disposal or sale of wastes or by-products. 

Gas cooler II 14. Tail gas scrubber 21, Final acid cooler... 

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