Sulfur trioxide absorption

Venturi concentrator The to be concentrated sulfuric acid is injected into the radiation scrubber and there brought into contact with a dry gas stream, which takes up and thereby removes the water vapor. The heat of evaporation is either supplied directly via hot gases (furnace gas) or indirectly by heating the acid to be concentrated (e.g. with tail gases from a double contact plant) or by heat exchange with hot sulfuric acid from sulfur trioxide absorption. In this preconcentration process waste heat can be utilized at low temperatures. The venturi concentrator is in particular employed when large quantities of dilute sulfuric acid or sulfuric acid strongly contaminated with solids has to be preconcentrated. 

The gas is now cooled and allowed to flow into the packed towers, where it is absorbed. The production of fuming sulfuric acid (oleum), however, requires sulfur trioxide absorption in special absorption towers irrigated with oleum. The reaction is exothermic. 

Heat exchange cooling between beds 1-2, 2-3 and 3 with sulfur trioxide absorption after bed 4. 

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. 

Phosphate fertilizer complexes often have sulfuric and phosphoric acid production facilities. Sulfuric acid is produced by burning molten sulfur in air to produce sulfur dioxide, which is then catalytically converted to sulfur trioxide for absorption in oleum. Sulfur dioxide can also be produced by roasting pyrite ore. Phosphoric acid is manufactured by adding sulfuric acid to phosphate rock. The... 

Absorption of sulfur trioxide in strong sulfuric acid... 

This reaction can be forced to effective complete conversion by first carrying out the reaction to approach equilibrium. The sulfur trioxide is then separated (by absorption). Removal of sulfur trioxide shifts the equilibrium, and further reaction of the remaining sulfur dioxide and oxygen allows effective complete conversion of the sulfur dioxide, Figure 6.6. 

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. 

Exothermic Gas Absorption with Complex Reaction Sulfonation and Discoloration in the Absorption of Sulfur Trioxide in Dodecylbenzene... 

The basic steps in the contact process are (1) production of sulfur dioxide (2) cooling and, for smelters, cleaning of the process gas (3) conversion of the sulfur dioxide to sulfur trioxide (4) cooling of the sulfur trioxide gas and (5) absorption of the sulfur trioxide in sulfuric acid.28 Figure 25.8 is a photograph of a contact process plant. A simplified diagram of a double absorption contact sulfuric acid process is shown in Fig. 25.9. Because sulfur dioxide is produced by several processes, it is convenient to separate the discussion of sulfur dioxide production from its conversion to sulfuric acid. 

Although the primary effects of sulfur dioxide are on the respiratory tract, inhaled sulfur dioxide can be transferred into the systemic circulation. After its rapid absorption, inhaled sulfur dioxide is rapidly converted to a mixture of sulfite, bisulfite, and sulfur trioxide (ATSDR 1998) ... 

Mann and Moyes111 developed an approximate film theory to describe gas absorption and interfacial temperature behavior under very exothermic conditions. They used the theory to analyze their own experimental data for the sulfur trioxide dodecylbenzene system. They showed (both experimentally as well as theoretically) that in a highly exothermic reaction system the chemical absorption rate could be lower than the physical absorption rate because the depression of interfacial solubility can greatly reduce the absorption potential under reacting conditions. 

Removal of sulfur dioxide from a gas stream. If a fuel that contains sulfur is burned, the product gas contains sulfur dioxide. If the gas is released directly into the atmosphere, the SO2 combines with atmospheric oxygen to form sulfur trioxide. The SO3 in turn combines with water vapor in the atmosphere to form sulfuric acid (H7SO4), which eventually precipitates as acid rain. To prevent this occurrence, the combustion product gas is contacted with a liquid solution in an absorption or scrubbing process. The SO2 dissolves in the solvent and the clean gas that remains is released to the atmosphere. 

In addition to the gases fisted in Table 3, Earth-based and spacecraft microwave spectroscopy indicates that H2SO4 vapor (with a mixing ratio of several tens of ppmv) is present below the clouds. Sulfur trioxide, as yet unobserved, is also expected to be present below the clouds in equilibrium with H2SO4 vapor. Spectrophotometers on Venera 11-14 found absorption of blue sunlight in Venus lower atmosphere. This is attributed to elemental sulfur vapor with a total mixing ratio (for all aUotropes) of —20 ppbv in Venus lower atmosphere. 

In the conventional contact process (single contact process) the reaction gases are passed through the four trays without intermediate absorption and the gas is cooled after each tray to 450°C or 430°C for the lowest tray. After passage through the first tray 60 to 63% of the sulfur dioxide has been converted to sulfur trioxide, after the second tray 89 to 90% and after the fourth tray a maximum conversion of 98% is possible, based on sulfur dioxide. 

Oxidation of sulfur dioxide with air, via the first stage of the contact or the chamber process to sulfuric acid, also serves to improve the collection efficiency of the sulfur oxides. Sulfur trioxide has a very strong affinity for water, unlike sulfur dioxide, so that its collection by direct absorption into water is extremely efficient, and the product sulfuric acid is a salable commodity. 

The sulfur trioxide concentration at this stage is about 10% by volume. After cooling to near ambient temperatures, this product is absorbed in concentrated or nearly concentrated sulfuric acid, where both absorption and hydration occur via countercurrent contact in a chemical stoneware packed tower (Eq. 9.26). 

Regeneration of high concentrations of sulfuric acid may also be achieved by addition of oleum or sulfur trioxide to diluted acid. The inventory of acid in circulation is increased by a corresponding amount (Eq. 9.26). A third method of reconcentration, useful when the acid consumption occurs as a part of, or adjacent to a contact sulfuric acid plant, is to pass the diluted acid itself through the acid plant absorption tower. This amounts to on-site addition of sulfur trioxide, and the increased acid inventory obtained can be sold to markets through the normal producer channels. 

For reactions with a positive AG° there are ways to increase the conversion. One standard method is to remove products in an intermediate step. This procedure is used in the double absorption contact process for sulfuric acid where sulfur trioxide is removed after the gas passes through two-packed bed reactors before entering the last two reactors. 

Sulfur Dioxide to Sulfur Trioxide Process. The manufacture of sulfuric acid involves the oxidation of elemental sulfur to SO2, followed by the catalytic oxidation of SO2 to SO3 over vanadium pentoxide. The next step involves the absorption of SO3 with water to form H2SO4. The SO2 oxidation reaction to... 

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