Sulfuric acid is an important chemical that is manufactured using the contact process. 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 with air in a combustion chamber, generating sulfur dioxide, oxygen, and nitrogen. The gases, at about 1900°F and at a pressure of about 50 in. WC, pass through a waste heat boiler generating saturated or superheated steam. The boiler could be of fire tube or water tube design. The gases are cooled to about 800°F, which is the optimum temperature for conver­sion of SO2 to SO3. The exit gas temperature from the boiler decreases as the load decreases.

In order to maintain the exit gas temperature at 800°F at varying loads, a gas bypass system is incorporated into the boiler, either internally or externally (Fig. 2.8). The gases then pass through a converter where SO2 gets converted to SO3 in a few stages in the presence of catalyst beds. The reactions are exothermic, and the gas temperature increases by 40-100° F. Air heating or superheating of steam is necessary to cool the gases back to 800°F. After the last stage of conversion, most of the SO2 has been converted to SO3. The gas stream containing SO3 gases at about 900°F is cooled in an economizer before being sent to an absorption tower. The flue gas stream is absorbed in dilute sulfuric acid to form concentrated sulfuric acid. The scheme is shown in Fig. 2.9. The steam thus generated in these waste heat boilers is used for process as well as for power generation.

The main boiler behind the sulfur combustor could be of fire tube or water tube design, depending on gas flow Extended surfaces may also be used if the gas stream has no dust. Sometimes, owing to inadequate air filtration and poor

Combustion, particulates are present in the flue gases, which could preclude the use of finned tubes. One has to be concerned about the casing design because of the possibility of sulfur condensation and corrosion. Soot blowing is not recommended, because it affects the gas analysis and adds moisture to the flue gases and may cause acid condensation.

Water-cooled furnace designs have an advantage in that the casing operates at the saturation temperature of steam, hence acid corrosion is unlikely. The main concern in sulfuric acid plants is corrosion due to acid condensation from moisture reacting with SO3. This is minimized by starting up and shutting down the plants on clean fuels if possible and avoiding frequent start-ups and shutdowns, which induce a cooler environment for possible acid condensation over the exchanger or economizer tubes. The boiler and exchanger casings must also be maintained above the dew point by using a ‘‘hot casing’’ design, which reduces the heat loss to the surroundings while at the same time keeping the casing hot, above 350-400°F, as required. Boilers may be kept in hot standby if frequent shutdowns and start-ups are likely.

The feedwater temperature as it enters the economizer has to be high, often above 320°F, to minimize acid dew point corrosion because the gas contains SO3. Carbon steel tubes with continuously welded solid fins have been used in several plants in the United States, whereas in Europe and Asia cast iron gilled tubes shrunk over carbon steel tubes are widely used. In a few projects, the sulfur deposits found their way between the gilled iron rings and the tubes and caused corrosion problems. The choice of tube materials is based on the preference and experience of the end user and the boiler supplier.

The internal gas bypass system increases the shell diameter compared to the external bypass system. The bypass pipe also cools the gases to some extent, so the damper is not exposed to the high temperature gases as in the external bypass system, where the damper is located in a refractory-lined pipe and handles the hot inlet gases. Operability and maintenance of the damper are important aspects of boiler operation. Both internal and external gas bypass systems have been used in the industry.

In fire tube boilers, ferrules and the refractory lining on the tube sheet protect the tube sheet from the hot gases. An external steam drum with down — comers and risers ensures adequate circulation of the steam-water mixture inside the shell.