SULPHURIC ACID

PROCESS DESCRIPTION

The manufacture of Sulphuric Acid is based on Double Conversion Double Absorption (DCDA) Process. The main steps consist of burning of elemental Sulphur in excess of air to form Sulphur Dioxide (SO2), conversion of Sulphur Dioxide to Sulphur Trioxide (SO3) in presence of Vanadium Pentoxide (V2O5) Catalyst and finally reacting Sulphur Trioxide, so formed, with water to produce a solution containing 98-98.5% H2SO4 (although from practical point of view, SO3 is absorbed in 98-98.5% H2SO4 to yield an acid of higher concentration which on dilution yields 98-98.5% Sulphuric Acid).

On account of the exothermic nature of the reactions in the Sulphur Burning, conversion & absorption, lot of heat is generated. This heat is partially utilised for generating steam at 15 kg/cm² g and partially to heat the gases up to the reaction temperature before entering the converter. The steam is also used for generating electricity.

Sulphur Circuit

Sulphur from Sulphur Storage Area is fed to the feeding Section of Sulphur Melter Pit where steam is passed through Helical Heating Coils to melt the Sulphur.
The temperature of Molten Sulphur is maintained by controlling the steam flow rate and molten sulphur is kept agitated by an agitator to avoid settling of impurities in melting section. Molten Sulphur Containing suspended impurities overflows into the settling section of Melter Pit. In this section impurities settle down while steam is passed through Hairpin heating coils to maintain the temperature of Molten Sulphur. Pure Sulphur from Settling Section then overflows into the clean Sulphur Section (or Pumping Section) of the Melter Pit. In this section, temperature of Molten Sulphur is maintained by passing steam through Hairpin heating Coils.

From Clean Sulphur pit, molten sulphur is pumped to the Sulphur Furnace using steam jacketed Sulphur Burner Feed Pumps. Molten sulphur is sprayed into Sulphur Burner through a Sulphur Gun and its flow is regulated by a manually operated, steam jacketed control valve.

Gas Circuit

In the burner, molten sulphur is burnt in excess of dry air to give Sulphur Dioxide (SO2). Air is fed to Sulphur Furnace through Air Blower after drying by scrubbing with Sulphuric Acid in Drying Tower.

The combustion gases from Sulphur Furnace are cooled down in Waste Heat Boiler-I and then passed through Hot Gas Filter to remove any dirt, dust etc. In Hot Gas Filter, quartz chips are used as filtering media.

Clean SO2 bearing gases are then fed to the first stage of Four Stage Converter where partial conversion of Sulphur dioxide (SO2) to Sulphur Trioxide (SO3) takes place in presence of Vanadium Pentoxide (V2O5) Catalyst. Conversion of SO2 to SO3 being exothermic, temperature of the gases in the Ist stage rises. Hot gases from Ist stage of converter are cooled down in Waste Heat Boiler-II before entering the IInd stage. By Product Steam at 15 kg/cm² g pressure is generated in the Waste Heat Recovery System.

After conversion in IInd stage of Converter, the gases are cooled down in Hot Heat Exchanger by heat exchange with cold gases from Intermediate Absorption Tower. The gases then enter IIIrd stage of converter.

The intermediate absorption is carried out after IIIrd stage of conversion. The gases from the IIIrd stage are first cooled down in Cold Heat Exchanger by gases coming from Intermediate Absorption Tower and then further cooled down in Economiser-I. Finally, gases from Economiser-I are absorbed in 98-98.5% Sulphuric Acid in the Intermediate Absorption Tower. Temperature of gases from Intermediate Absorption Tower is raised by passing the gases through Cold Heat Exchanger and then Hot Heat Exchanger before entering the IVth stage of converter.

Final absorption is carried out after the IVth stage of conversion. Gases from IVth stage are cooled down in Economise-II and then absorbed in 98-98.5% Sulphuric Acid in Final Absorption Tower. The absorption efficiency at both stages of absorption is about 99.9%. Flue gases within the pollution control limits are vent off from Final absorption Tower through Stack.

Acid Circuit

Sulphuric Acid from all the three towers flows by gravity to the Process Acid Tank where it is diluted by DM Water to get required concentration of 98-98.5%.

The acid from Process Acid Tank at 98-98.5% concentration is cooled in Acid Cooler and is fed continuously to three towers) by vertical Process Acid Pumps. Product Acid of 98-98.5% concentration is withdrawn continuously from the outlet stream of Acid cooler through Product Acid Cooler after being cooled to 40-45°C, for storage in Acid Storage Tanks.

Water / Steam Circuit

DM Water is used as boiler feed water in Waste Heat Recovery System and for dilution of concentrated Sulphuric Acid in Process Acid Tank.

DM Water from DM Water Plant is first collected in DM Water Tank from where it is pumped by Deaerator Feed Water Pumps to Boiler Feed Water Storage Tank through Deaerator. Dissolved gases are removed in Deaerator using saturated steam at a pressure of 0.5 kg/cm² g from Pressure Reducing Station.

Sulphite from Sulphite Dosing Tank is mixed with DM Water in the suction of the Deaerator Feed Water Pumps if required. From the Deaerator, the Deaerated water is then pumped to Economiser-I by Boiler Feed Water Pumps Phosphate from phosphate Dosing Tank if required, is mixed with Deaerated water in the suction of the Boiler Feed Water Pumps. Both Sulphite & Phosphate dosing is done in order to remove residual oxygen and to prevent scale formation respectively as and when required.

After heat removal of IIIrd stage & IVth Stage Converter gases in Economiser-I & Economiser-II respectively, water at elevated temperature alongwith 13-14% steam enters the Steam Drum.

From the Steam Drum, the boiler feed water enters the Waste Heat Boiler-I & II. On removal of heat from Sulphur Burner exit gases and Ist stage converter gases in Waste Boilers, saturated steam at 15 kg/cm² g is produced which flows to Steam Drum .

Pressure of steam from Steam Drum is first reduced to 8 kg/cm² g through Pressure Reducing Station after which part of the steam is sent to Sulphur Melting Pit and remaining quantity of steam is sent to Pressure Reducing Station where after reducing its pressure to 0.5 Kg/cm² (g), steam is sent to Boiler Feed Water Storage Tank & to Steam Condenser.

Steam Condensate from Condensor is then taken into Condensate Tank from where it is pumped to the DM Water Storage Tank with the help of Condensate Return Pumps for reusing as boiler feed water.

Process water from Plant Battery Limits is used as raw water in water softening unit and DM Water Plant after passing it through a series of Activated Carbon Filter & Sand Filter. From the Water Softening Unit, soft water is fed to the Cooling Tower basin to make up the evaporation & drift losses. From Cooling Tower, Cooling Water is then distributed, through Cooling Water Pump, to Acid Cooler, Product Acid Cooler , Steam Condenser and to Instrument Air Compressors.

LIST OF EQUIPMENT

Production Capacity : 100 TPD

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