The stack gases of the refinery are scrubbed with an aqueous solution of ammonia in column C-101 (see Figure 8-3). The ammonia solution is introduced into the column at the top, while flue gases enter the column near the bottom. The rich effluent containing dissolved sulfur oxides, called sulfitic brine, enters tank TK-101, where any dust contained in the stack gases is separated out by settling.
The sulfitic brine is pumped by P-101 to heat exchanger E-101, heated by medium-pressure steam, and flashed in drum V-101, where partial decomposition of sulfites take place at 300°F and 30 psia pressure. The gases produced as a result of decomposition of ammonium sulfites, S02
SUBMERGED COMBUSTION REACTOR R-101
SUBMERGED COMBUSTION REACTOR R-101
Figure 8-3. Flue gas desulfurization. B.F.W. = boiler feedwater.
and NH3, are sent to R-103, the conversion reactor. The concentrated brine is sent by P-102 to R-101.
The concentrated brine, which consists of all ammonium sulfates, thio-sulfates, and unconverted sulfites, is fed by metering pump P-102 to a sulfate decomposition reactor R-101, which is a bath of molten ammonium sulfates, where sulfate reduction takes place. Also, a stream of molten sulfur is pumped by metering pump P-106 to R-101. The temperature of the molten bath is maintained at 660°F. The reactions in the sulfates reduction reactor are endothermic. The heat is supplied to the system by submerged flame combustion of fuel gas and air. The hot gases are sent from a special burner directly into the molten bath via a graphite tube. This combustion takes place under slightly reducing conditions, implying that air/fuel gas inlet ratio is maintained slightly lower than stoichiometric (90%). The reduction of sulfates start at 570-590°F. The operating temperature is not allowed to exceed 750°F, when ammonia starts to react with medium. The operating temperature of the bath can be adjusted according to the thiosulfate/sulfate ratio in the bath. The molten salt in the bath is very corrosive. The reactor is therefore lined with acid-resisting refractory.
The gases above the molten bath (80% NH4HSO4 and 20% (NH4)2 S04) contain a certain amount of S03 due to vapor pressure. The S03 is reduced to S02 by the following exothermic reactions. The CO and H2 originate from unburned fuel gas. The reactions are carried over a solid state (Co-Mo on alumina) catalyst:
The off gases from reactor R-102 at about 700°F are quenched to 375°F by injection of boiler feedwater.
The S02-rich gases coming from V-101, R-101, and R-102 are mixed with H2S from an outside source, such as a refinery amine plant, through compressor K-102 and enter conversion reactor R-103 near the bottom. The H2S/S02 feed ratio must be maintained higher than 2, to protect the chemical system.
The reactor is an absorber column with 13 perforated trays. The solvent polyethylene glycol (PEG) is circulated by pump P-103 through cooler E-103 to remove the heat of reaction, feed enthalpy, and to keep the tower temperature at 300°F. Heat is removed by thermosyphon recirculation of boiler feedwater through E-103 and raising LP steam.
The PEG solvent flows down the column. The Claus reaction takes place in a liquid solvent PEG (polyethylene glycol, mol. wt. 400). Ammonia in the gas stream acts as a catalyst. The molten sulfur produced is collected by decantation at the bottom of the reactor. Makeup solvent is added to the cooling loop to compensate for the solvent losses.
As the Claus reaction in R-103 does not reach 100% completion, gases coming out from R-103 overhead contain NH3, H2S, S02, C02, inerts, sulfur vapors, and entrained PEG. These are quenched with boiler feedwater and sent to thiosulfate drum V-102. Water quenching lowers the temperature to the dew point of the gases. S02 reacts with sulfur in the presence of NH3 to give thiosulfates, which are dissolved in water. This solution, also containing traces of PEG, is sent back by P-105 to the gas line coming from R-102. The temperature being higher than 190°F, the thiosulfates are cracked into gaseous S02, NH3, and sulfur.
The off gases from V-102 at approximately 200°F are cooled in exchanger E-102. Most of the water is condensed and collected in drum V-103 with some ammonia, H2S, and C02 dissolved in it.
The top gases from V-103 are washed in drum V-105 by bubbling contact with brine from the ammonia scrubber bottom to further reduce pollutant release into atmosphere.
The following reaction takes place:
The operating conditions and feed composition of a flue gas desulfur-ization unit are shown in Table 8-12. The feed and product compositions and utility consumption are shown in Tables 8-13 to 8-15.
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