Tail gas scrubbing

The background for the development of VK69 was a need for reduction of S02 emissions from double-absorption plants by installing a more active catalyst at low temperature downstream from the intermediate absorption tower. Clearly, the catalytic solution should be more competitive than the alternatives, e.g. tail gas scrubbing or triple-absorption layout, in terms of capital and operating costs. In the following, the required technical performance of the catalyst with respect to S02 oxidation activity, mechanical strength and pressure drop is discussed, and input from the literature and from practical experience in the field is presented. Reviews of the extensive literature published on sulphuric acid catalysts can be found in [2-5],... 

Sulfuric Acid Options - To meet current standards, sulfuric acid plants must generally be designed as double-contact, doubleabsorption plants (6), or they must use a tail-gas scrubbing step to generate an alternate product, e.g., ammonium sulfate. 

Tail gas scmbbers are sometimes used on single absorption plants to meet S02 emission requirements, most frequently as an add-on to an existing plant, rather than on a new plant. Ammonia (qv) scrubbing is most popular, but to achieve good economics the ammonia value must be recovered as a usable product, typically ammonium sulfate for fertilizer use. A number of other tail gas scrubbing processes are available, including use of hydrogen peroxide, sodium hydroxide, lime and soda ash. Other tail gas processes include active carbon for wet oxidation of S02, molecular sieve adsorbents (see MoLECULARSIEVEs), and the absorption and subsequent release of S02 from a sodium bisulfite solution. 

In the case of low/variable percentage SO2 in the gases, a single absorption process with a good tail gas scrubbing system operating continuously is a better option. 

Various designs are used for the tail gas scrubbing system. 

Note with tail gas scrubbing there are practically no emissions of sulfur dioxide in modern plants SO3 + H2SO4 expressed as SO3 When sulfur burning... 

Temperature control is accurate and easy. Combustion of gaseous fuel generally does not cause pollution unless it contains sulphurous compounds. Tail gas scrubbing system shall be installed when such gases are used. 

Alternatively, the ammonium sulfite-bisulfite solution may be acidulated with phosphoric acid, producing an ammonium phosphate solution. There are nurnerous other tail-gas scrubbing processes that produce various byproducts or waste products [5]. 

Electrostatic precipitators are very efficient however, they have a high capital cost. In specific locations final tail-gas scrubbing with ammonia or caustic soda solutions is necessary. 

All of these changes may account for about 20%-25% of the investment costs of a new installation. Tail-gas scrubbing is the lowest cost option (15% of the total revamping cost) however, this generates a liquid effluent that needs to be processed. 

Single absorption is also used but achieves lower conversion at 98.0 to 98.5 per cent, and emissions of SO2 in tail gas are much higher (five times). Unless tail gas scrubbing is applied, single absorption plants will generally not meet normal environmental emission standards. 

Environmental emission of SO2 can be controlled by tail-gas scrubbing. This, although efficient, adds considerably to running costs. In this process SO2 is absorbed by lime or A,iV-dimethylaniline. 

A similar installation was employed by Dobratz et al. (1933), who evaluated three difieient absorption tubes in the cooler-absorber (IM-in. ID Karbate, 1-in. ID tantalum, and 0.88-in. ID stainless steel) and investigated the production of 36 to 40% hydrochloric acid solutions. Hiese authors presort complete data, including heat-transfo and absorption-coefficient correlations for the cooler-absorber, but give no data on the performance of their 4-in. by 4-ft packed tail-gas scrubbing tower. 

Any SO2 not oxidized during catalytic SO2 oxidation is not condensed to H2S04(f). It must be scrubbed from the gas before leaving the WSA plant. Tail gas scrubbing is described in Chapter 28. 

Figure 28.2 Acid plant tail gas scrubbing. Tail gas enters the scrubber and is contacted with a solution that removes SO2. The saturated, near S02-free, tail gas exits the scrubber through a stack.

Figure 28.3 Gas-liquid contacting tower designs commonly used for acid plant tail gas scrubbing. Large arrows represent gas flow in each figure. Packed towers are not commonly used for high scaling potential solutions such as those containing lime or limestone. Copyright 2013 MECS Inc. All rights reserved. Used with permission of MECS Inc.

Tail gas scrubbing processes convert SO2 into forms that are considered wastes or by-products. Table 28.2 provides an overview of acid plant tail gas scrubbing processes. The reactions shown in the table all occur in solution as shown above. 

Tail gas scrubbing with ammonia is mostly used for sulfur burning acid plants located at fertilizer production sites. An ammonia solution is used to remove SO2 from the tail gas producing solid ammonium sulfate as a by-product that is sold as fertilizer. 

The continued worldwide lowering of ambient SO2 concentration standards is expected to further increase the use of acid plant tail gas scrubbing as an effective means of decreasing acid plant SO2, SO3, nitrogen oxides, and H2SO4 mist emissions. 

Chapter 28 described industrial acid plant tail gas scrubbing. Typical achievable tail gas SO2 concentrations for various technologies are shown in Table 29.5. The key advantage of tail gas scrubbing is that its performance does not inherently decrease in time as with catalyst. 

Table 29.5 Summary of common acid plant tail gas scrubbing processes and their expected tail gas SO2 concentrations (Brown, 2011)...

Tail gas scrubbing technology Tail gas SO2 concentration (ppmv)... 

---