Double absorption sulfuric acid process

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. 

More recentiy, sulfuric acid mists have been satisfactorily controlled by passing gas streams through equipment containing beds or mats of small-diameter glass or Teflon fibers. Such units are called mist eliminators (see Airpollution control methods). Use of this type of equipment has been a significant factor in making the double absorption process economical and in reducing stack emissions of acid mist to tolerably low levels. 

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... 

DOT safety regulations, 25 337 Double absorption process, for sulfuric acid manufacture, 23 769 Double absorption sulfuric acid plants, 23 774... 

At present, the most common method used in sulfuric acid production is the double-absorption process based on oxidation of sulfur dioxide to siifur trioxide on vanadium pentoxide catalyst. Tliis process will remain the prevalent method of sulfuric add production in the future. New sulfuric acid installafions will be aimed mainly at increasing the output from production units, maximum heat recovery, and operational rellabilify of installation, particularly through precise selection of extra-resistant alloys and spedal noncorrosive materials. Further increases in SO2 to SO3 conversion effi-denqy would do little to improve emissions because the h h efficiency of present processes already achieves very low SO2 emissions. 

Modern sulfuric acid plants use the double absorption process to reach a SO2 conversion of >98%, which is needed to meet current environmental standards. The intermediate removal of SO3 by absorption after the third bed enables a conversion of SO2 after the fourth bed of >99%. 

The single absorption contact process for sulfuric acid is characterized by four main process steps gas drying, catalytic conversion of SO2 to SO3, absorption of SO3, and acid cooling. The maximum SO2 conversion for a single absorption plant is about 97.5 to 98 percent. By adding another SO3 absorber, the SO2 conversion can be increased to 99.5 to 99.7 percent, resulting in lower SO2 emissions. The so-called double absorption process is now the industry standard. 

A block diagram for tlie double-absorption sulfuric acid process is gi en in Fig. 8.6.1. Atomized molten sulfur is burned in a horizontal, brick-... 

Figure 8.6.1. Conceptual diagram of typical double-absorption sulfuric acid process.

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. 

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 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. 

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. 

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

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). 

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. 

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. 

At present the target technology is a sulfuric acid plant wnth double conversion and double absorption with mist eBmination. The 99.5% sulfur recovery may be achieved by this process structure. Hiis gives a release of 3.3 kg S02/tonne of 100% sulfuric acid. 

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