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| Ammonium Sulphate production from coke oven gas containing ammonia |
Coke oven gas with a pressure of 2500 mm WC from exhauster is fed to ASP. Ammonia present in co gas is recovered in ASP as ammonium Sulphate fertilizer. By simple approximation of unit operation the process can be described as follows.
PROCESS:
The co gas from the exhauster is fed to pre-heat the gas to 60-700C. But according to our atmospheric temperature, this temperature is obtained after the exhauster. So the gas is directly fed to the saturator.
Saturator is a cylindrical vessel with a conical bottom. It is provided with a bubbler hood, which is duct prolonged to the middle of the saturator. The duct has a hood at the bottom provided with vanes like arrangement. Another ring-like structure with small openings is at the conical portion, which is used for nitrogen feeding. Hot water rings are provided at the top of saturator. Saturator is always maintained with acid bath called liquor, which contains 4-5% of sulphuric acid. The coke oven gas enters through the bubbler hood which is dipped in the bath. The gas rises through the mother liqueur. During this period, the ammonium present in the gas reacts with the sulphuric acid in the liquor. The following reactions will take place in the saturator
NH3 + H2SO4 → NH4 (HSO4)
Ammonium Sulphate thus formed settles at the bottom of saturator. Pure nitrogen is purged into the saturator through N2 rings at 6-7 kg/cm2. N2 purging increases crystal growth. Pure Sulphuric acid (98%) is fed to the saturator to maintain the acidity in the saturator. The gas collected at top of the saturator is fed the acid trap. As the gas rises up, some of the crystal may be carried with the gas and they get stacked to walls of the saturator at top. Then the hot water is sprayed to ring provide. The crystal attached to the walls of saturator is washed away. When hot water has sprayed the concentration of the liquor decreases. So inlet acid concentration is increased to 6-7% at that period. After the reaction, the mother liquor is continuously drawn to circulating tank provided at the side of the saturator. This acts as a seal for saturator. From the circulating tank, the mother liquor is fed to the mother liquor tank. The crystals collected at the bottom are fed crystal receiver tank by using a pump
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ACID TRAP:
The outlet of the saturator carries some acid mist. In order to remove the acid Mist, the gas is sent to the acid trap. It is a hollow cylindrical vessel. The coke oven gas from saturator enters tangentially to the trap. Due to the centrifugal motion, the acid mist gets separated. The acid collected a bottom is fed to the circulating tank. The Coke oven gas is fed to the Benzol recovery.
CRYSTAL RECEIVER TANK:
Ammonium slurry from the bottom of the saturator is pumped to crystal receiver tank with conical bottom the ammonium Sulphate crystal settled at the conical portion of the tank, which is wet liquor. The mother liquor from the top of the receiver is fed to the saturator. The slurry from the top of the receiver is fed the saturator. The slurry from the bottom is fed to centrifuge.
DRIER:
Drier is a Design of Fluidized Drier. The principle is based on the loose materials property to acquire fluidity in the air flow under a definite air velocity. The crystal from centrifuge contains some amount of moisture. To remove this moisture crystal are to be dried. The drier is provided with a screen at the bottom, ceramic rings are arranged at the bottom of the screen. The drier is provided with forced draught fan and air, heated in the duct. A spreader at the feed chute of the drier spreads the feed in all directions.
Forced draught fans suck the atmospheric air and feed to the drier. The discharge chute of the fan is divided into two sections. The air is heated to 120 -150 deg C by using the steam and the hot air is feed from the bottom of the screen. The ceramic rings distribute the air in all directions and allow the crystals in a fluidized state. The temperature of the air is sufficient moisture of the crystals. At the discharge end of the drier, cools the crystals.
When the pressure level of the fluidized bed reaches the set point (300-400mm Wc) an automatic discharge feeder discharge the dry ammonium Sulphate to the bucket elevator. The elevator discharges the dry product into the bunker, which in turn feeds the product to the bagging machine. The zone above the fluidized bed is kept under 5-10mmWc in order to avoid the carry-over of the ammonium Sulphate particles out of the drying unit to the dust catcher.
CYCLONE SEPARATOR:
The air from the drier is sucked by the suction fan and feed to the cyclone separators. Cyclone separators separate fine ammonium Sulphate crystals in the air and feed to the bunker. The air from the cyclones is feed to the dust collecting tanks which contains flushing liquor up to certain level. The dust-laden air is then fed to the bottom of the tank. The crystals then dissolved in the water and the air is vented into the atmosphere.
MOTHER LIQUOR TANK:
The excess liquor from the saturator enters the mother liquor tank. Each saturator is provided with two mother liquor tanks. One is vertical and is horizontal. First, the Liquor enters the horizontal tank. As the liquor has less density than the tar it floats. Then the clear mother liquor from the bottom is feed to the vertical tank. From the bottom of the vertical tank, the mother liquor is fed to the saturator through the pumps provided. The concentration of the liquor is maintained at 10-12 %. If the concentration decreases, the density of the liquor decreases and it may be contaminated. Then crystals may become back.
NITROGEN SUPPLY FOR AGITATION
Nitrogen flow to the saturator is normally 450- 550 m3/h, at nitrogen pressure before saturator 0.5- 0.7kg/cm2. For saturator stopping, reduce the nitrogen flow to 100m3/h. once the gas valves are vents unsaturated, stop the nitrogen flow completely.
Coke oven gas is a product of coal coking in C.O. battery its composition is as follows based approximate design of plants.
Hydrogen - 50 - 60;
Methane - 25 -33;
Carbon dioxide - 7-8;
Carbon monoxide - 2.2-3;
Nitrogen - 3-4;
Oxygen - 0.2-0.8;
Unsaturated compounds - 2-2.5
Ammonia vapor - 6-12g/m3;
Hydrogen supplied - 6-12 g/m3