LITERATURE REVIEW There are various methods to be considering in the industrial production of sulphuric acid. When analysing the most efficient production method several aspects should be thoroughly considered. This includes the economic, environmental and the social aspects of production. production. It is through compromising several levels of these factors that you must choose the most efficient and feasible means of sulphuric acid production. THE CONTACT PROCESS The Contact Process is an effective m ethod of producing sulphur dioxide dioxide in three steps. The initial step is to make sulphur dioxide. This is normally done by burni ng sulphur in an excess of air or by heating sulphide ores in an excess of oxygen.About 70% of sulphuric acid is made from elemental elemental sulphur.(Davenport sulphur.(Davenport and King 2006, pg. 5). Other options include metallurgical offgas, t hat includes sulphur dioxide, is able able to be used for sulphuric acid production once the dust is removed from from the gas, usually by electrostatic precipitatio precipitation. n. If the gas is not removed, gas flow can be stopped. 20% of sulphur dioxide is produced this way ( Davenport and King 2006, pg 5,7). The third option to making sulphur dioxide is spent acid regeneration, when sulphuric acid which has been used as a catalyst is regenerate regenerate d by catalysing oxidizing oxidizing the offgass SO2 to SO3.10% of sulphur dioxide is produced produced this way (D avenport and King 2006, pg. 7). The nextstep, is the reversible and exothermic conversion of sulphur dioxide into sulphur sulphur trioxide. This step occurs at high temperatures and low pressures to consider both equilibrium equilibrium and rate .SO 2 rich gases enter a catalytic converter, usually a tower with m ultiple catalyst catalyst beds, and get c onverted to SO 3. The exit gases from this stage contain both SO 2 and SO3 which are passed through an absorption towers where sulphuric sulphuric acid is dripped down columns.The catalytic converter converter and absorpt ion tower steps may be repeated. The final step is converting converting the sulp hur trioxide into into sulphuric acid . This step involves dissolving sulphuric acid to make oleum so that the sulphur dioxide is more controllable when when you proceed to the next action, of adding water. The adding of water produces sulphuric sulphuric acid (Clark 2002). In this production process, the equilibrium needs to be shifte d towards the forward re action, the production of sulphur trioxide. trioxide. As this is exothermic, low temperatures f avour t he forward reaction. 2SO 2(g) + O 2(g) 2SO 3(g)
H° « -100 MJ per kg-mole of SO 3 (Davenport and King 2006, pg 2)
However, the two types of catalysts used which are effective under usual gas gas conditions, vanadium and platinum, platinum, have are typically not active below below 370°C -420°C ( Donovan and McAlister1982pg 212). This is the most significant constraint constraint on this process, as these temperatures do not favour the forward reaction. A catalyst must be used because of the slow reaction time, and the most commercially used catalyst is vanadium pentoxide pentoxide (Rinckhoff 198 2, pg 35). Low temperat ures also shift the reaction, but this slows the reaction down. When manufac turing sulphur dioxide, it is not efficient to produce an equilibrium mixture containing a high proportion of sulphur trioxide if it takes an excessive amount of time. Gases need to reach equilibrium within the short time they are in contact wi th the catalyst reactor.Following Le Chateliers Principle, Principle, if you increase the pressure of the system during the oxidation of sulphur dioxide to sulphur trioxide trioxide , it will favour the conversion of sulphur trioxide. Therefore, high pressures pressures will favour the forward reaction yet is not economically feasible so are performed at low pressures. Even at low pressures close to atmospheric, there is a 99.5% conversion of sulphur dioxide into sulphur trioxide. trioxide. ( Clark 2002). The increase of conversion from increasing the pressure isnt worth the increase in cost of production. Higher pressures then 1 bar would need all equipment equipment to be designed as pressure vessels vessels for safety reasons (Louie 196 1, pg. 3-3). The second step, the conversion of sulphur sulphur dioxide dioxide to sulphur trioxide, needs to take place place in the presence presence of a catalyst. A single absorption sulphuric sulphuric acid plant consist of one or more catalyst beds arranged in series followed by an absorption tower to absorb the sulphur trioxide formed. After sulphur dioxide is oxidised to sulphur trioxide, the sulphur trioxide is absorbed by concentrated sulphuric acid in absorbers, preceded by oleum absorbers where necessary. In the absorbers, the sulphur trioxide trioxide is converted to sulphuric sulphuric acid by water in the absorber Conversion efficiencies of 95% to 98% sulphur dioxide to sulphur sulphur trioxide are typical depending depending on the numbe r of catalytic beds (Louie 196 1, pg. 3-3).In the double contact process, a primary conversion efficiency of 80% to 9 3%, depending on the arrangement of the c ontact beds and of contact time, is obtained obtained in the primary contact stage of a converter preceding the intermediate intermediate absorber. (Afghan Energy, Chemical & Mining Industries Industries 2004 ) After cooling the the gases, the sulphur trioxide trioxide already already formed is absorbed in th e intermediate absorber in sulphuric acid. The absorption of the sulphur trioxide brings about a considerable considerable shift in the reaction equilibrium equilibrium towards the formation of SO 3 because residual sulphur dioxide has been removed from the mixture, resulting in considerably higher overall conversion efficiencies. (Louie 196 1, pg. 3 -3) The sulphur trioxide formed in the secondary stage is absorbed in t he final absorber.In general, SO 2 feed gases containing up to 12 Vol.% SO 2 are used for this process. The conversion efficiency in new plants can rea ch about 99 .6%. (Afghan Energy, Chemical & Mining Industries Industries 2004).
Double contact acid making is more efficient than single contact acid making. However,this extra efficiency is accompanied with extra costs ; additional heat exchangers, energy for moving gas and acid through the sec ond absorber and the cost of a second absorbing tower. The double absorption process requires more capital than single absorption yet shows significant operating cost advantages (Friedman 1982 ). However, the costs a re a compromise needed to be made to increase the efficiency of the conversion. Dual absorptionhas generally been acc epted as the best available control technology for meeting NSPS emission limits acid (Inorganic Chemical Industry no d ate). There are no by -products or waste scrubbing materials created, only additional sulphuric trioxide. So in addition to the dual contact sulphuric acid process is not only efficiently feasible but is environmentally feasible. A double contact sulphuric acid production process is environmentally feasible as there are no by -products and emissions are low. Due to low damage t o the environment and the safety of production the general public could accept such a process. The double contact sulphuric acid production has a very high conversion rate of 99 .6% and is very efficient, although through being technically and efficiency feasible, there is additional costs generated. The downfall in the economic aspect though, is a comprise that is more than reasonable to consider.
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REFERENCES Davenport, W G and King, M J 2006, Sulfuric Acid Manufacture: Analysis, Control and O ptimization, Elsevier Ltd., UK Donovan, J. R. and McAlister, D. R. 1982, Sulphuric Acid Plants: R & DReport, in Chemical Engineering Progress (editors), Sulfuric/Phosphoric Acid Plant Operations , American Institute of Chemical Engineers, United States of America, New York , pp 212-215 Rinckhoff, J. B. 1 982, Natures Law for Sulfuric Acid Plants, in Chemical Engineering Progress (editors), Sulfuric/Phosphoric Acid Plant Operations , American Institute of Chemical Engineers, United States of America, New York, pp33-40 Friedman, J. L. 1982, Sulfuric Acid Energy Design for the 80s, in Chemical Engineering Progress (editors), Sulfuric/Phosphoric Acid Plant Operations , American Institute of Chemical Engineers, United States of America, New York, pp101-107 Jim Smith 2002, Chemguide, United Kingdom, accessed 02/03/2 011 Louie, D. K. 1961, Handbook of Sulfuric Acid Manufacturing , DKL Engineering Inc., Canada Inorganic Chemical Industry no date, US EPA, United States of America, accessed 05/03/2011 Afghan Energy, Chemical & Mining Industries 2004, Nawabi , Afghanistan, accessed 09/0 3/2011
