Absorption towers acid cooling

Sulfuric acid plants need water that has three qualities demineralized water for boiler feed, process makeup water to absorb SO3, and cooling water. The water treatment installations do not produce harmful wastes. Recycled process condensate or demineralized water is-used-for the absorption tower. The cooling water amount and quality depend on the type of heat exchangers used and water available. 

Gas leaving the converter is normally cooled to 180—250°C using boiler feedwater in an "economizer." This increases overall plant energy recovery and improves SO absorption by lowering the process gas temperature entering the absorption tower. The process gas is not cooled to a lower temperature to avoid the possibiUty of corrosion from condensing sulfuric acid originating from trace water in the gas stream. In some cases, a gas cooler is used instead of an economizer. 

Gas leaving the economizer flows to a packed tower where SO is absorbed. Most plants do not produce oleum and need only one tower. Concentrated sulfuric acid circulates in the tower and cools the gas to about the acid inlet temperature. The typical acid inlet temperature for 98.5% sulfuric acid absorption towers is 70—80°C. The 98.5% sulfuric acid exits the absorption tower at 100—125°C, depending on acid circulation rate. Acid temperature rise within the tower comes from the heat of hydration of sulfur trioxide and sensible heat of the process gas. The hot product acid leaving the tower is cooled in heat exchangers before being recirculated or pumped into storage tanks. 

Downstream from the 3rd bed, the gas is cooled and passed to an intermediate absorption tower, in which the S03 formed is absorbed in recirculating sulphuric acid. The cold and practically S03-free process gas is reheated to 380-440°C and returned to the converter, where the remaining SO2 is converted to S03 in a 4th catalyst bed. The rest of the S03 is subsequently recovered in a final absorption tower before the process gas, containing a small fraction of unconverted S02, is emitted through the stack. The combustion air is dried with the 98 wt% product acid in order to avoid corrosion and acid mist problems in the plant. The sulphuric acid process normally operates close to atmospheric pressure with the combustion air blower dimensioned just for compensation of the pressure drop through the plant. 

In this process (Fig. 1), the reactor contains a rhodium-platinum catalyst (2 to 10% rhodium) as wire gauzes in layers of 10 to 30 sheets at 750 to 920°C, 100 psi, and a contact time of 3 X 10"4 second. After cooling, the product gas enters the absorption tower with water and more air to oxidize the nitric oxide and hydrate it to nitric acid in water. Waste gases contain nitric oxide or nitrogen dioxide, and these are reduced with hydrogen or methane to ammonia or nitrogen gas. Traces of nitrogen oxides can be... 

In the process (Fig. 1), sulfur and oxygen are converted to sulfur dioxide at 1000°C and then cooled to 420°C. The sulfur dioxide and oxygen enter the converter, which contains a catalyst such as vanadium pentoxide (V205). About 60 to 65% of the sulfur dioxide is converted by an exothermic reaction to sulfur trioxide in the first layer with a 2 to 4-second contact time. The gas leaves the converter at 600°C and is cooled to 400°C before it enters the second layer of catalyst. After the third layer, about 95% of the sulfur dioxide is converted into sulfur trioxide. The mixture is then fed to the initial absorption tower, where the sulfur trioxide is hydrated to sulfuric acid after which the gas mixture is reheated to 420°C and enters the fourth layer of catalyst that gives overall a 99.7% conversion of sulfur dioxide to sulfur trioxide. It is cooled and then fed to the final absorption tower and hydrated to sulfuric acid. The final sulfuric acid concentration is 98 to 99% (1 to 2% water). A small amount of this acid is recycled by adding some water and recirculating into the towers to pick up more sulfur trioxide. 

Most plants use reactors with various stages in order to cool the stream for the catalytic step. Conversion by a vanadium pentoxide catalyst deposited on a silicate support is the critical step in the process, in which the gaseous stream is passed over successive layers of catalyst. The gas mixture is then passed through an absorption tower. Oleum, the product, is a concentrated solution of sulfuric acid containing excess sulfur trioxide. 

The acid condensate resulting from the cooling of NO-containing combustion gases is fed into the absorption towers... 

The absorber consists usually of a single stainless-steel tower, although two or more may be necessary for low-pressure plants. Another kind of absorber used is a drum absorber. In general, the cool gases along with additional air are fed to the bottom of the tower, and demineralized water is fed to the top of the tower. Instead of demineralized water, clean condensed steam can be used. Nitric acid is taken out from the bottom, whereas tail gases leave at the top of the tower. The absorption towers can be packed with sieve plates, bubble cap plates, or turbo-grids. 

Gases from the outlet of the CHE/Economiser after CHE are taken to a 25% oleum tower before IPAT. The circulating oleum is boiled to produce SO3 vapors which are absorbed in a glass/Teflon-lined steel (MS-PTFE) absorption tower. A glass acid cooling system is generally employed. The unabsorbed gases are led to IPAT. 

In a variant to the above process, provision is made by means of a glass constmc-tion/MS-PTFE absorption tower (with glass internals—tower packing) parallel to the final absorption tower along with the glass acid cooling system. 

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