Sulfuric acid 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. 

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... 

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 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. 

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. 

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... 

Practically all processes of sulfuric acid production presently in use are based on catalytic oxidation of sulfur dioxide to sulfur trioxide and subsequent absorption in recirculating sulfuric acid to form H2SO4. The process of oxidation is accomplished most commonly in multibed converters on vanadium pentoxide catalyst. 

Based on elementary sulfur, three consecutive reactions are involved. First, sulfur is oxidized with air to SO2. Subsequently, SO2 is further oxidized catalytically to sulfur trioxide, and finally sulfuric acid is formed by absorption and reaction of SO3 in/with water. In contrast to sulfur oxidation to SO2, the oxidation to SO3 is limited by thermodynamic constraints, and is the crucial reaction step in H2SO4 production. Oxidation of sulfur in air is carried out in a refractory-lined furnace. Sulfur dioxide is also produced as a by-product of roasting of sulfide ores such as ZnS or FeS2. 

The industrial production of sulfuric acid is based on the absorption of sulfur trioxide in water following the reaction... 

In another method, chlorosulfonic acid is manufactured by addition of hydrogen chloride to chlorosulfonic acid containing sulfur trioxide (7 wt%) in a steel reaction tower to react with 93% of the sulfur trioxide. Part of the resultant crude chlorosulfonic acid was recycled to the SO3-absorption tower. The remainder of the crude product was treated with hydrogen chloride in a teflon-lined steel reactor tower to produce chlorosulfonic acid [1500 kg hour", containing iron (0.3 ppm)]. ... 

Sulfuric acid plants are designed with optimized catalyst volumes and bed inlet temperatures to give a reasonable approach to equilibrium in each bed to achieve the maximum possible conversion of sulfur dioxide to sulfur trioxide. As shown by the examples in Table 2.8, this results in a significantly smaller volume in bed 1 than the remaining beds. The total catalyst volume used normally corresponds to a loading of 180-220 liters of catalyst per tonne of sulfuric acid produced per day although many plants use more, depending on conditions and the source of the sulfur dioxide. Lower volumes of catalyst are normally used in double-absorption units. 

Other impurities vary widely. One common impurity is water, which can be removed by either absorption or adsorption. Another is ammonia (NH3), which is basic, rather than acidic. Sulfur trioxide (SO3), prussic acid (HCN), and nitrogen oxides (NO, ) are of concern because of their high chemical reactivity. Oxygen must be removed from some reagent streams, and nitrogen can be absorbed to upgrade natural gas. 

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