Sulfuric acid process, double-contact

Contact Sulfuric Acid Process Monsanto, Parsons, Davy Powergas, others Can accept elemental sulfur, or H2S and S02-bearlng streams down to about 5Z sulfur content A double contact/double absorption plant can recover up to 99.8Z of the sulfur fed to it. All sulfur compounds handled... 

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. 

In the early 1970s, air pollution requirements led to the adoption of the double contact or double absorption process, which provides overall conversions of better than 99.7%. The double absorption process employs the principle of intermediate removal of the reaction product, ie, SO, to obtain favorable equiUbria and kinetics in later stages of the reaction. A few single absorption plants are stiU being built in some areas of the world, or where special circumstances exist, but most industriali2ed nations have emission standards that cannot be achieved without utili2ing double absorption or tad-gas scmbbers. A discussion of sulfuric acid plant air emissions, control measures, and emissions calculations can be found in Reference 98. 

Implementation of cleaner production processes and pollution prevention measures can yield both economic and environmental benefits. The following production-related targets can be achieved by measures such as those described above. The numbers relate to the production processes before the addition of pollution control measures. In sulfuric acid plants that use the double-contact, double absorption process, emissions levels of 2 to 4 kilograms of sulfur dioxide... 

IPA [Interpass absorption] Also called Double absorption, and Double catalysis. An improved version of the Contact process for making sulfuric acid, by which the efficiency of the conversion of sulfur to sulfuric acid is increased from 98 percent to over 99.5 percent. 

The contact process was invented by Phillips in England in 1831 but was not used commercially until many years later. Today 99% of all sulfuric acid is manufactured by this method. It was developed mainly because of the demand for stronger acid. All new contact plants use interpass absorption, also known as double absorption or double catalysis. This process will be described in detail in Fig. 2.3. 

Fig. 25.9. Schematic flow diagram of a double absorption contact process for sulfuric acid manufacture.

The process is called double contact acidmaking because gas and sulfuric acid are contacted twice, steps (b) and (d). 

Sulfur dioxide is converted to SO3 and ultimately to sulfuric acid using the double contact process, which is based upon the equihbrium between SO2 and SO3 (equation 12). 

The oxidation of sulfur dioxide to sulfur trioxide and its subsequent conversion to sulfuric acid is currently almost exclusively carried out using the contact process, in particular the double contact proce.ss. The lead chamber process is no longer important. 

With the double contact process it is unnecessary to purify the tail gases to reduce their sulfur dioxide content still further, whereas tail gases from single contact plants have to be purified. This can be realized either by scrubbing with ammonia or with an aqueous solution of sodium sulfite and sodium hydrogen sulfite (Wellman-Lord process), absorption on activated charcoal (sulfacid process from Lurgi) or by oxidative gas purification such as in the peracidox process (oxidation of sulfur dioxide with hydrogen peroxide or peroxomonosulfuric acid). 

The emission of sulfur dioxide from sulfuric acid plants is strongly reduced with the double contact process. If all the sulfuric acid manufactured in the Federal Republic of Germany were produced by modern plants using the double contact process, the resulting emission of sulfur dioxide would account for only 0.32% of the total emission from human activities. 

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. 

Alkylation conditions are quite mild. Processes employing concentrated sulfuric acid operate at 2-12°C and 4.1—4.8 x lO N/m (4.1-4.8 atm), with a gas contact time of 5-30 min. Catalysis with anhydrous hydrogen fluoride requires slightly higher temperatures (25 5°C) and pressures than sulfuric acid, and contact times of 20-40 s. Both catalysts involve initial protonation of the double bond of an olefin. With isobutylene, for example, -butylcarbonium ion is formed. This can add to another isobutylene in the same manner as occurs during polymerization (Eq. 18.25). 

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. 

The means for reducing the sulfur dioxide emissions from contact sulfuric acid plants are already relatively well developed. Any sulfur dioxide recovered as such can be recycled to the acid plant for conversion to acid. The most popular approach, at least in new plants, appears to be the double-contact process (59, 60) which is simply an extension of the basic contact process itself. In sulfur-burning plants, this presents no serious problems, but where the acid plant is operating on a relatively dilute process gas, it may not be possible to operate autogenously (10). In the latter instance, the double-contact process can still be used, but auxiliary heat must be supplied (61). Some forms of the double-contact process are reported capable of operating autogenously with sulfur di-... 

A sulfuric acid plant will use the double-contact, double-adsorption (DC/ DA) process [8] to get an overall SO2 conversion of 99.5% for a feed with 11% SO2 and 10% O2. The first converter will operate at about 1.5 atmo-... 

The Spanish Civil War and World War II finally ruined the Spanish pyrites industry. Shipments had been blocked during these years, and alternatives had been found. After World War II, many new sulfuric acid plants were constructed in Europe to replace those that had been destroyed, and U.S. expansion was bolstered by economic growth, especially by demand for phosphate fertilizers. These new plants all used elemental sulfur (Contact process). While Spanish pyrites production returned to pre-war levels by 1950 (see Figure 2.5 for the early history of production), their market share had seriously eroded as sulfur demand, overall, had more than doubled. Pyrites mining as a source of sulfur continued in Spain until 2002. 

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