Almost all sulfuric acid is manufactured by the contact process.
Properties of Sulfuric Acid
- Molecular formula : H2SO4
- Molecular weight : 98.08gm/mole
- Appearance : Water white slightly viscous liquid
- Boiling point : 2900C
- Melting point : 100C
- Density : 1.840gm/mL (liquid)
- Solubility : Miscible with water in all proportions
- Viscosity : 26.7cP (200C)
- Aqueous sulfuric acid solutions are defined by their H2SO4 content in weight-percent terms.
- Anhydrous (100%) sulfuric acid sometimes referred to as monohydrate, which means that it is the monohydrate of SO3.
- Dissolve any quantity of SO3, forming oleum (fuming sulfuric acid).
- The physical properties of sulfuric acid and oleum are dependent on H2SO4and SO3 concentrations, temperature, and pressure.
Sulfuric Acid Raw Materials
Basis: 1000kg sulfuric acid (100%)
Sulfur dioxide or pyrite (FeS2) = 670kg
Air = 1450-2200Nm3
Sources of raw material
The sources of sulfur and sulfur dioxide are as follows
- Sulfur from mines
- Sulfur or hydrogen sulfide recovered from petroleum desulfurization
- Recovery of sulfur dioxide from coal or oil-burning public utility stack gases
- Recovery of sulfur dioxide from the smelting of metal sulfide ores
2PbS + 3O2 ----> 2PbO + 2SO2
- Isolation of SO2 from pyrite
Sulfuric Acid Reactions
Sulfuric Acid Process Flow Diagram:
Figure: Manufacturing of Sulfuric acid by Contact process
Steps in the sulfuric acid Contact Process
The steps in this process are as follow.1. Burning of sulfur
2. Catalytic oxidation of SO2 to SO3
3. Hydration of SO3
1. Burning of sulfur
Burning of sulfur in presence of dry air is carried out in sulfur pyrite burner. As SO2 is needed for the catalytic oxidation and prevention of corrosion, dry air is used in the combustion process. If sulfur contains carbonaceous impurities, the molten material has to be filtered to avoid poisoning the catalyst and forming water from burning hydrogen.
2. Catalytic oxidation of SO2 to SO3
When using sulfur from sources 1 and 2, purification of the SO2 gas is normally not needed. Other sources of SO2 require wet scrubbing followed by treatment of the gas with electrostatic precipitators to remove fine particles. The catalyst used is vanadium pentoxide (V2O5) and the pressure is 1.2-1.5 atmospheres. The temperature has to be kept around 4500C. If it rises above 4500C, the equilibrium is displaced away from SO3. Temperature should reach around 4500C for the catalyst to be activated. This process is strongly exothermic. The catalytic reactor is designed as a four-stage fixed-bed unit. The gas has to be cooled between each step. Four passes, together with "double absorption, described below, are necessary for overall conversion of 99.5-99.8% (three passes, 97-98%). The temperature rises to over 6000C with the passage of the gas through each catalyst bed. The doubled absorption consists of cooling the gases between each bed back to the desired range by sending them through the heat exchanger and then back through the succeeding beds. Between the third and fourth beds, the gases are cooled and sent to anabsorption tower. This is to shift the equilibrium to the right by absorbing SO3. The gases are then sent to the heat exchanger to warm them to 410-4300C and then on to the fourth catalyst bed.3. Hydration of SO3
After the catalytic oxidation process, the resulting SO3 is hydrated by absorption in packed towers filled with 98-99% sulfuric acid. This is the H2SO4 azeotrope of minimum total vapour pressure. The catalytic oxidation has to proceed in high yield to avoid air pollution problems. SO2 has a low solubility in 98% H2SO4. At lower acid concentrations, sulfuric acid and SO3 form a troublesome mist and at higher concentrations emissions of SO3 and H2SO4 vapour become significant. The absorption acid concentration is kept within the desired range by exchange as needed between the H2SO4 in the drying acid vessel that precedes the combustion chamber with the H2SO4 in the absorption tower. The acid strength can be adjusted by controlling the streams of H2SO4 to give acid of 91 to 100% H2SO4 with various amounts of added SO3 and water. The conversion of sulfur to acid is over 99.5%.Operation of multistage convertor:
Figure: Multistage reactor for the conversion of SO2 into SO3
The apparatus in which SO2 is converted into SO3 is as shown in figure. It is designed so as to achieve high rate of conversion along with highest possible thermodynamic yields. The convertor is subdivided into several compartments having separate layers of catalytic mass supported by meshes. In four compartment reactor, upon entering the reactor from top , the sulfurous gases have been heated to about 4000C by heat exchange carried out earlier on the sulfurous gases themselves, the added air or the mixture of them are heated up to about 6000C where upon they react. The rate of reaction is high but the yield does not exceed 75%.
Upon leaving the first compartment the temperature of the partially converted gases is lowered by 1000C in the gas-gas heat exchanger (HE-1), and they are returned to the converter where, in correspondence with the temperature of the catalytic bed in the second compartment, they are brought up to about 5000C and react to form further SO3 from SO2. The rate of reaction is lower but the yield goes up to 85%.The gases are again sent out of the reactor and their temperature is reduced again by 1000C by means of heat exchanger (HE-2). Then returned to third compartment where yields raised up to 95% by passing through the catalytic bed at 4800C. The rate of reaction is further lowered, but now only small amounts of gas to be converted into SO3.
After lowering the temperature third time by external heat exchange (HE-3), the gases are passed back to the reactor where they undergo on the catalytic bed in the fourth compartment, final conversion at about 4500C, which gives yield of 98-99%.
Sulfuric Acid Major engineering problems
- Design of multistage catalytical convertor for highly exothermic reaction. Earlier two stage converter is used but nowadays the design of three or four stages rather than conventional two stage operation are developed.
- To optimize space velocity in catalyst chamber because it deals with pumping cost or fixed charges of reactor
- Thin catalyst beds of 30-50cm height used to avoid above difficultties. Yield can drop due to longitudinal mixing if the convective gas velocity through the bed is low
- Removal of heat of absorption of SO3 in acid. Pipe coolers with water dripping over external surface have been replaced by cast iron pipe with internal fins to promote better heat transfer.
- Pressure drop must be low, so, 8cm stacked packing is often used.
Uses of Sulfuric Acid
- The largest single use is in the fertilizer industry.
- Mostly in production of phosphoric acid, which in turn used to manufacture fertilizers such as triple superphosphate, mono and diammonium phosphates
- Used for producing superphosphate and ammonium sulfate.
- Used as an acidic dehydrating reaction medium in organic chemical and petrochemical processes involving such reactions as nitration, condensation, and dehydration, as well as in oil refining, in which it is used for refining, alkylation, and purification of crude-oil distillates
- In the inorganic chemical industry e.g. in the production of TiO2 pigments, hydrochloric acid, and hydrofluoric acid
- In the metal processing industry e.g. for pickling and descaling steel, for leaching copper, uranium, and vanadium ores in hydrometallurgical ore processing, and in the preparation of electrolytic baths for nonferrous-metal purification and plating
- Certain wood pulping processes in the paper industry require sulfuric acid, used in textile and chemical fiber processes and leather tanning
- In manufacture of explosives, detergents and plastics
- In production of dyes, pharmaceuticals