Sulphuric Acid Production by Dcda Process

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INTRODUCTION

Sulphuric Acid is an important chemical, which has large-scale industrial uses. Its major user is the phosphate fertilizer industry. Other important applications are in petroleum refining, steel pickling, rayon & staple fiber,alum, explosives, detergents, plastics and fibers etc. Sulphuric Acid Industry is very old and has been continuously adopting the technological developments. It started with Lead Chamber process followed by contact rocess with Single Conversion Single Absorption (SCSA) and now Double onversion Double Absorption Process (DCDA) process.

In India, there are about 140 Sulphuric Acid Plants (130 Sulphur based & 10 Smelter Gas based) with Annual Installed Capacity of about 12 Million MT.As in some places, there are more than one stream/unit with some common facilities these plants are situated in 131 locations. It may be observed that about 25% of the installed capacity is located in Paradeep (Orissa) only consisting of 2 plants (4 streams/units). The current annual production of Sulphuric Acid is about 5.5 Million MT,against the installed capacity of 12 Million MT/Annum from Sulphur based as well as Smelter Gas based plants. Some of the plants have been closed down due to various reasons. It is learnt that many captive plants have been closed down due to non-viability of the main plant, viz. Single Super Phosphate etc., while merchant plants have closed down due to poor demand. The demand of Sulphuric Acid is fully met by the current production, as the installed capacity is more than double the demand.

The contact process for the production of Sulphuric Acid is based on the catalytic oxidation of SO2 to SO3. Since the oxidation reaction of SO2 to SO3 is exothermic, the temperature of the gases after catalytic conversion rises. As the forward reaction progresses, the state of equilibrium is approached and the forward reaction comes to halt. In DCDA Plant, SO3 is removed from the gas stream after 2nd and 3rd bed, which shifts the equilibrium and increases the rate of the forward SO2 to SO3 reaction resulting in higher overall conversion.

STATUS OF EXISTING SULPHURIC ACID PLANTS
Process Technology
Technical features of manufacturing process technology of Sulphuric Acid Plants with regard to pollution control aspects are discussed below: The primary raw material for producing intermediate product of sulphurdioxide is elemental sulphur. Nonferrous metals industry is also a large producer of sulphur dioxide where the roasting and smelting processes generate off-gases with a sufficiently high concentration of sulphur dioxide that could be converted to sulphuric acid as a byproduct.Significant number of sulphuric acid manufacturing facilities has been installed in metallurgical plants for the recovery of SO2, mainly for environmental reasons. Pyrite is also used as a raw material for sulphuric acid production. The theoretical requirement of Sulphur is 0.3265 MT/MT of 100% Sulphuric Acid. However, the actual consumption in DCDA Sulphuric Acid Plants generally varies from 0.330 MT to 0.340 MT/MT of 100% Sulphuric Acid depending on the SO3, and acid mist escaping with the stack gases. The sulphur loss in fugitive emission is generally negligible.Sulphuric Acid Plants vary in a number of respects depending on the raw material used to produce the gas containing sulphur dioxide. The broadest division is into hot gas and cold gas plants. In hot gas plants, which are usually based on the combustion of elemental sulphur, the hot gas from the sulphur furnace is cooled just to the required converter inlet temperature. Typical process flow sheet for Sulphur burning DCDA Plant is given in figure.Cold gas plants, based on metallurgical or decomposition gases, are so designed that the crude sulphur dioxide containing gas must be cooled to low temperatures in the de-dusting and cleaning systems before being introduced into the sulphuric acid plant. It is therefore, necessary to reheat the cold feed...
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