Double absorption 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. 

The curve in Figure 21 represents SO2 equiUbrium conversions vs temperature for the initial SO2 and O2 gas concentrations. Each initial SO2 gas concentration has its own characteristic equiUbrium curve. For a given gas composition, the adiabatic temperature rise lines can approach the equiUbrium curve but never cross it. The equiUbrium curve limits conversion in a single absorption plant to slightly over 98% using a conventional catalyst. The double absorption process removes this limitation by removing the SO from the gas stream, thereby altering the equiUbrium curve. 

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. 

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

Tail gas emissions are controlled by improving the S02 conversion efficiency and by scrubbing the tail gas. In a double absorption process plant, a five-bed converter has 0.3 percent unconverted S02, as compared with 0.5 percent for a four-bed converter. A Lurgi Peracidox scrubber may be used to remove up to 90 percent of the residual S02 in the tail gas from a double absorption plant. Hydrogen peroxide or electrolytically produced peroxymonosulfuric acid is used to convert the S02 to H2S04 in the Lurgi scrubber. 

The Contact Process was further modified to the Double Contact Double Absorption process, which uses Le ChateUer s principle. According to this, when the SO3 produced (as a result of conversion of SO2 by the catalytic process) is absorbed by the intermediate absorption towers, the equilibrium conversion shifts to the right, i.e., more SO2 is converted to SO3. This is accomplished by reheating and then passing the reaction gases through an additional bed of catalyst. The SO3 produced (additionally) is then absorbed in a second absorption tower. 

Removal of SO3 (double contact double absorption process) from the reaction zone. 

The principal disadvantage of the pressure contact process compared with the conventional double-absorption process is that it consumes more power. 

Double absorption processes based on metallurgical gases differ from hot-gas plants based on sulfur combustion in that cold feed gases must be heated to the converter-inlet temperature using the energy liberated in the oxidation of sulfur dioxide. 

In case of sulphuric acid plants, the earlier single-conversion single-absorption process was replaced by the double-conversion double-absorption process for most of the new plants. The SO2 % in gases at converter inlet was increased from 7.5-8.0 % to 11.0-11.5 % which could reduce the volume of gases to be handled for the same production rate. The power required by the air blower got reduced as a result. 

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. 

It is known that the conversion of sulphur dioxide to the trioxide is encouraged by high pressure. ICI have developed a process in which conversion is carried out at 5-7 atmospheres in a three-bed converter at the temperatures for the double absorption process. In this case single absorption is used (Figure 3.14) and 99% conversion is achieved. 

As already examined for the isothermal and the optimal case (Figures 6.3.7 and 6.3.8), we now calculate the residence time for four adiabatic beds for the double absorption process with intermediate absorption after the third. The plots of l/rso2 vs. Xso2 are shown in Figure 6.3.11. For each bed, the residence time is given by Eq. (6.3.22), if we use 550 °C as mean temperature ... 

Figure 63.10 SO2 conversion versus temperature plot for four adiabatic beds for single and double absorption process with intermediate absorption for comparison, the optimal pathway (dot-dashed line) is also shown (1 bar feed gas 8% SOj, 11% O2, rest N2).

Here we obtain a residence time for the double absorption process (Xso2 = 99.6%) of 3.4 s. 

Four adiabaticaHy operated beds with intermediate cooling and absorption of SO3 after the third bed (double absorption process, Figures 6.3.10-6.3.12) 3.4 99.6... 

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. 

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