Sulfur double contact

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. 

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... 

Sulfuric Acid Options - To meet current standards, sulfuric acid plants must generally be designed as double-contact, doubleabsorption plants (6), or they must use a tail-gas scrubbing step to generate an alternate product, e.g., ammonium sulfate. 

Fig. 1.4. Double contact sulfuric acid manufacture flowsheet. The three main S02 sources are at the top. Sulfur burning is by far the biggest source. The acid product leaves from two H2SO4 making towers at the bottom. Barren tail gas leaves the final H2S04 making tower, right arrow.

Fig. 9.1. Single contact H2SO4 making flowsheet. SO3 rich gas from catalytic SO2 oxidation is reacted with strong sulfuric acid, Reaction (1.2). The reaction consumes H20(f) and makes H2S04(f), strengthening the acid. Double contact H2SO4 making is described in Fig. 9.6. A few plants lower the SO2 content of their tail gas by scrubbing the gas with basic solution (Hay et al., 2003).

Most sulfuric acid plants are double contact plants, Fig. 9.6, Tables 9.3, 19.3 and 23.2. They efficiently oxidize their feed S02(g) to S03(g) and efficiently make the resulting S03(g) into H2S04(f). Single contact plants (Fig. 9.1) are simpler and cheaper - but less efficient. 

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

Fig. 19.1. Schematic of 3-1 double contact sulfuric acid plant. The plant consists of ...

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 65 to 70% sulfuric acid containing a residue of metal sulfates (3 to 5%) is either directly, or after further concentration to 80 to 90%, in admixture with 96% acid or oleum (from a double contact plant fed with sulfur dioxide from metal sulfate cracking) is further utilized in the digestion of titanium ore. 

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. 

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. 

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 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 conversion efficiencies of double-contact systems reportedly range from 99.5% to as high as 99.9%, with exit sulfur dioxide concentrations ranging from about 500 ppm to as low as 100 ppm (60, 62). These conditions obviously depend on the initial gas conditions and system design factors. 

SO2 from the sulfuric acid plant Double contact acid plant, Ammonia-separator, High-speed catalyst Valuable byproduct (add). Ammonium sulfate as byproduct... 

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-... 

Mercury is removed in a Boliden-Norzinc tower. The acid plant is of the double contact type and its production capacity is 900 t-concentrated sulfuric acid daily. A part of the SO2-containing gas is sent to a liquid SO2 plant. 

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. 

Figure 11.3. Sulfur-Bunting Double-Contact, Double Absorption Sulfuric Acid Plant.

---