Tail gas scrubbing processes

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. 

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]. 

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. 

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

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. 

Other methods such as absorption by silica gel or molecular sieves or scrubbing with alkalies or urea solutions are also used by some companies but to a smaller extent. Detailed descriptions of tail-gas treatment processes are given in Chapter 19 [11,12]. 

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. 

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. 

Trencor Also called Trentham Trencor. A wet-scrubbing process for removing residual sulfur dioxide and hydrogen sulfide from the tail gas from the Claus process. 

Satriana (2) provides a summary of the development of flue gas treatment technology. The first commercial application of flue gas scrubbing for sulfur dioxide control was at the Battersea-A Power Station [228 MW(e)] in London, England, in 1933. The process used a packed spray tower with a tail-end alkaline wash to remove 90 percent of the sulfur dioxide and particulates. Alkaline water from the Thames River provided most of the alkali for absorption. The scrubber effluent was discharged back into the Thames River after oxidation and settling. A similar process was also operated at the Battersea-B Power Station [245 MW(e)] beginning in 1949. The Battersea-B system operated successfully until 1969, when desulfurization efforts were suspended due to adverse effects on Thames River water quality. The Battersea-A system continued until 1975, when the station was closed. 

Figure 25.4 shows a typical sulfur recovery plant based on the Claus process. The tail gas from the Claus reactors may be further processed to remove any remaining sulfur compounds. Combined H2S removal efficiencies of 99.5-99.99 percent are achievable.20 This may be done by low-temperature Claus-type solid-bed processes (e.g., the Sulfreen process), wet-Claus absorption/oxidation processes (e.g., the Clauspol 1500 process), or hydrogenation of the off-gas to form H2S for recycle (e.g., the SCOT process). Residual sulfur compounds in the tail gas are then incinerated to S02. The residual S02 in the oxidized tail gas may be scrubbed by any of several processes (e.g., the Wellman-Lord process) before being vented to the environment. It is feasible to bring the H2S content of... 

Tail gas emissions are controlled by improving the S02 conversion efficiency and by scrubbing the tail gas. In a double absorption process plant, a five-bed converter has 0.3 percent unconverted S02, as compared with 0.5 percent for a four-bed converter. A Lurgi Peracidox scrubber may be used to remove up to 90 percent of the residual S02 in the tail gas from a double absorption plant. Hydrogen peroxide or electrolytically produced peroxymonosulfuric acid is used to convert the S02 to H2S04 in the Lurgi scrubber. 

In the process, 99.8 percent ethylene, 99.5 percent oxygen, and recycle gas are directed to a vertical reactor and are contacted with the catalyst solution under slight pressure. The water evaporated during the reaction absorbs the exothermic heat evolved, and make-up water is fed as necessary to maintain the catalytic solution concentration. The reacted gases are water-scrubbed and the resulting acetaldehyde solution is fed to a distillation column. The tail gas from the scrubber is recycled to the reactor. Inerts are eliminated from the recycle gas in a bleed stream which flows to an auxiliary reactor for additional ethylene conversion. 

Zeolites have also proven applicable for removal of nitrogen oxides (NO ) from wet nitric acid plant tail gas (59) by the UOP PURASIV N process (54). The removal of NO from flue gases can also be accomplished by adsorption. The Unitaka process utilizes activated carbon with a catalyst for reaction of NO, with ammonia, and activated carbon has been used to convert NO to N02, which is removed by scrubbing (58). Mercury is another pollutant that can be removed and recovered by TSA. Activated carbon impregnated with elemental sulfur is effective for removing Hg vapor from air and other gas streams the Hg can be recovered by ex situ thermal oxidation in a retort (60). The UOP PURASIV Hg process recovers Hg from clilor-alkali plant vent streams using more conventional TSA regeneration (54). Mordenite and clinoptilolite zeolites are used to remove HQ from Q2, clilorinated hydrocarbons, and reformer catalyst gas streams (61). Activated aluminas are also used for such applications, and for the adsorption of fluorine and boron—fluorine compounds from alkylation (qv) processes (50). 

The Electrochemical Membrane Separator (EMS) technology being developed is compared to a wet removal process with subsequent Claus Plant processing to elemental sulfur and SCOT Tail Gas treatment of flue gases. This wet process utilities aqueous Methyldiethanolamine (MDEA) as an absorbent in a scrubbing operation to bring the HjS level from 1.7% to 4 ppm. The flow rate to these processes is 50MM SCFD and the process pressure is 715 psi. The... 

Alkali for tail gas scmbbers and effluent treatment plant ( TP) (to take care of any sudden load due to process upset), and gas scrubbing systems must be in working order, and not bypassed. 

The ammonia and other volatile nitrogen compounds are then removed from the coal gas by water scrubbing. To assure over 98% sulfur removal and recovery, the trace amounts of COS in the coal gas are converted to H S via catalytic hydrolysis. The HjS is then removed from the coal gas by the conventional Dow GAS/SPEC ST-1 methyldiethanolamine (MDEA) process and the is con-verted to valuable liquid sulfur by the conventional Claus process with MDEA-based tail gas treating. 

Table 28J Overview of scrubbing processes used to treat acid plant tail gas (after Hay et al., 2003)... 

Clauspol [Claus polyethylene glycol] A variation on the Claus process for removing hydrogen sulfide from gas streams, in which the tail gases are scrubbed with polyethylene glycol to remove residual sulfur dioxide. Clauspol 150 is a modification of this. Developed by the Institut Frangais du Petrole. 

With the double contact process it is unnecessary to purify the tail gases to reduce their sulfur dioxide content still further, whereas tail gases from single contact plants have to be purified. This can be realized either by scrubbing with ammonia or with an aqueous solution of sodium sulfite and sodium hydrogen sulfite (Wellman-Lord process), absorption on activated charcoal (sulfacid process from Lurgi) or by oxidative gas purification such as in the peracidox process (oxidation of sulfur dioxide with hydrogen peroxide or peroxomonosulfuric acid). 

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