Heat Recovery in Sulfuric Acid Plant
Heat Recovery in Sulfuric Acid Plant
Sulfuric acid is an important chemical that is manufactured using the contact process (Figure 4.16). Heat recovery plays a significant role in this system, whose main objective is to cool the gas stream to a desired temperature for further processing. Raw sulfur is burned
in air in a combustion chamber generating SO 2 ,O 2 ,N 2 (see Table 4.2 for gas analysis). The flue gas at about 1040°C and at a pressure of 1200 mm wc passes through a waste heat boiler generating saturated or superheated steam at medium pressure. The boiler could be fire tube or water tube. The flue gas is cooled to about 425°C, which is the optimum tem-
perature for conversion of SO 2 to SO 3 . The exit gas temperature from the boiler decreases as the load decreases. In order to maintain 425°C for the reaction to occur, a gas bypass system is incorporated in the boiler. This can be external or internal as shown in Figure 4.3.
The flue gas then goes to a converter where SO 2 gets converted to SO 3 in a few stages in the presence of catalysts. The reactions are exothermic, and the gas temperature increases
Drum
Superheated steam Feed water
Scheme for sulfuric acid manufacture. Note: (1) Sulfur combustion furnace, (2) waste heat boiler, (3) contact apparatus, (4) superheater, (5) economizer, and (6) absorption tower.
182 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers
by 20°C–50°C. Air heating or superheating of steam is necessary to cool the gases back to 425°C. After the last stage of conversion, almost all of the SO 2 is converted to SO 3 . The gas stream at 480°C containing SO 3 gases is cooled in an economizer before being sent to an absorption tower. The flue gas is absorbed in dilute sulfuric acid to form concentrated sulfuric acid as shown in Figure 4.16. The steam generated in the boiler and superheater is used for power as well as process.
The main boiler behind the sulfur combustor can be of fire tube or water tube design, depending on gas flow. Extended surfaces may also be used if the gas stream has little or no dust. At the hotter end, plain tubes are used followed by tubes with low fin den- sity, and then as the gas cools, tubes with higher fin density are used. Sometimes due to inadequate air filtration and poor combustion, particulates are present in the flue gas, which could preclude the use of finned tubes or cause fouling if finned tubes are used. Casing can be of membrane wall design or refractory lined. The advantage of membrane wall casing is that acid condensation concerns are eliminated as the casing operates at the saturation temperature of steam, which is typically well above the acid dew point. With refractory-lined casing, the heat loss from the casing will be slightly more compared to membrane wall design; however, it is likely to be slightly less expen- sive. One has also to be concerned about the casing design due to the possibility of low-temperature corrosion if there are cracks in the refractory. The main concern is
corrosion due to acid condensation from moisture reacting with SO 3 . This is minimized by starting up and shutting down the boiler on clean fuels and avoiding frequent start- ups and shutdowns. If refractory lined, a hot casing design is preferred as discussed in Chapter 6 on miscellaneous calculations. Boiler may be kept in hot standby mode also to minimize acid condensation by injecting steam using a sparger pipe inside the lower drum or placing heating coils inside the drum. Soot blowing is not recommended as it affects the gas analysis and adds moisture to the flue gas. The boiler tubes are generally of carbon steel construction.
The feed water temperature to the economizer must be high above 160°C–170°C to minimize the condensation of acid vapor. If the water vapor content is zero, then the acid condensation concern may not be there, but there is always leakage of air into the system, which can introduce moisture into the system. Carbon steel tubes with carbon steel–welded solid fins have been used in many plants in the United States for the econo- mizers, while in Europe and Asia, cast iron gilled tubes shrunk over carbon steel tubes are widespread.
The internal gas bypass system increases the shell diameter compared to the external bypass system. The internal bypass system has a damper in the bypass section as well as in the main tube section, and by modulating the two dampers, the desired final gas tem- perature is reached. The advantage over the external bypass system is that due to some cooling in the bypass and the main tubes, the dampers located at the boiler exit operate in
a slightly cooler gas temperature range compared to the external bypass system. An exter- nal steam drum with risers and downcomers ensures adequate circulation and cooling of the tube sheet.