Double-contact process

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). 

Puritlcation of tail gases can be dispensed with in the double contact process... 

Double contact process reduces the sulfur dioxide emission by almost a factor of ten... 

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. 

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-... 

The double contact process including double absorption is shown in the block block diagram in Chapter 1 with the raw material—sulfur. In general, SO2 feed gases containing up to 12 vol.% SO2 are used for this process. The conversion efficiency in new plants can reach about 99.6% as a daily average in the case of sulfur burning. 

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... 

Maximize the recovery of sulfur by operating the furnaces to increase the SO, content of the flue gas and by providing efficient sulfur conversion. Use a double-contact, double-absorption 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. 

Figure 6.12 Cross-sectional view of the 4H-SiC power BJT fabrication, (a) Starting epilayer structure. (b) Dry etching of emitter and base epilayers. (c) p implantation for guard rings and contacts to p-base. (d) Formation of ohmic contacts, (e) Over layer metal deposition, (f) Double metal process.

Second lau analysis is applied to a 100 tonnes per day double-contact double-absorption (DC-DA) sulphuric acid plant in order to bring out true energy conversion efficiencies and consumptions based on uork availability of various streams. Second lau efficiencies are compared uith those of first lau to pinpoint true losses and inevitable consumptions in energy conversion processes. 

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.

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

Widespread industrial adoption of double contact acidmaking indicates, however, that the high efficiency of the process more than offsets these extra costs. 

Double contact acidmaking is more efficient than single contact acidmaking. This has made it the most used industrial process. The reason for its high efficiency is its efficient oxidation of S02 in its after-H2S04-making catalyst bed(s), Chapter 19. 

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