Sulphuric acid double absorption

Figure 1. Simplified flow sheet for a sulphur-burning double-absorption sulphuric acid plant (3+1 layout).

Figure 7.8 Sulphur-burning double absorption sulphuric acid process...

Note that the sulphur trioxide is not absorbed directly by water, as the reaction is very exothermic and a corrosive mist of droplets of concentrated sulphuric acid is formed above the mixture. A double absorption sulphuric acid plant allows a 99.5% conversion of sulphur dioxide to sulphur trioxide to take place. 

Sulphuric acid is the largest volume chemical in the world with an annual production of about 180 mill, t/year which is used primarily for phosphate fertilizers, petroleum alkylation, copper ore leaching and in smaller quantities for a number of other purposes (pulp and paper, other acids, aluminium, titanium dioxide, plastics, synthetic fibres, dyestuffs, sulphonation etc.). The major sulphur sources for sulphuric acid production are sulphur recovered from hydrocarbon processing in the refineries and from desulphurisation of natural gas, SO2 from metallurgical smelter operations, spent alkylation acid, and to a minor extent mined elemental sulphur and pyrites. A simplified flow sheet of a modem double-absorption plant for sulphuric acid production from sulphur is shown in Fig. 1. 

The background for the development of VK69 was a need for reduction of S02 emissions from double-absorption plants by installing a more active catalyst at low temperature downstream from the intermediate absorption tower. Clearly, the catalytic solution should be more competitive than the alternatives, e.g. tail gas scrubbing or triple-absorption layout, in terms of capital and operating costs. In the following, the required technical performance of the catalyst with respect to S02 oxidation activity, mechanical strength and pressure drop is discussed, and input from the literature and from practical experience in the field is presented. Reviews of the extensive literature published on sulphuric acid catalysts can be found in [2-5],... 

VK69 was introduced in the market by Haldor Topsoe in 1996 and has subsequently been installed in the final passes of more than 50 double absorption sulphuric acid plants worldwide. Due to the superior activity of VK69, the S02 emissions can typically be reduced by a factor 2 and new double absorption plants can be designed with less than 40 ppm S02 emission [17], The industrial experience also confirms low deactivation rate and low screening loss as expected from the results obtained in the development phase. 

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. 

Fig. 25.11. Sankey energy flow diagram for a 1000ton/day sulfur-burning double absorption sulfuric acid plant (feed gas 10% S02). A Blower B Sulphur furnace C Waste heat boiler D Catalyst bed 1 E Steam superheater F Catalyst bed 2 G Boiler H Catalyst bed 3 J Intermediate heat exchangers K Intermediate absorber L Converter bed 4 M Economizer N Final absorber O Air dryer P Acid coolers. (Courtsey Lurgi GmbH, Frankfurt, Germany.)...

The Zincor process is based on the roast-leach-electrowinning process. The zinc sulphide concentrate is roasted in four Lurgi fiuo-solids roasters to produce zinc oxide and sulphur dioxide gas. The SO2 gas is fed to two Monsanto double absorption acid plants. 

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. 

Process engineers are thus seen to have been able to provide practical solutions to the problems of reducing atmospheric emissions in sulphuric acid plants. They have also achieved a further halving of both wastage and pollution by designing plants that operate at pressure. Increased pressure favours increased conversion because there is a volume contraction on reaction. In France a double absorption plant has been operated at 4 bar for over ten years. 

Thionyl chloride method. Mix 100 g. of pure p-nitrobenzoic acid and 126 g. (77 ml.) (1) of redistilled thionyl chloride in a 500 ml. round-bottomed flask. Fit the flask with a double surface reflux condenser carrying a calcium chloride (or cottou wool) guard tube and connect the latter to an absorption device e.g., Fig. II, 8, 1. c). Heat the flask on a water bath with occasional shaking for 1 hour or until the evolution of hydrogen chloride and sulphur dioxide ahnost ceases. Allow the reaction mixture to cool, transfer it cautiously to a Claisen flask connected with a water-cooled condenser and a receiver (compare Fig. II, 13, 1). Distil off the excess of thionyl chloride (b.p. 77°) slowly and continue the distillation until the temperature rises rapidly to about 120° this will ensure that all the thionyl chloride is remov. Allow to cool, and distil the residual p-nitrobenzoyl chloride under diminished pressure as detailed in the Phosphorus Pentachloride Method. The resulting p-nitrobenzoyl chloride (a yellow crystalline solid) weighs 107 g. and melts at 72-73°. 

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