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NITROGEN FERTILISERS / AMMONIUM NITRATE PRILLING TOWER – SELECTIVE FILTRATION OF ONLY MOST HEAVILY CONTAMINATED AIR
PROCESS INFORMATION
Maximum gas flow of 235,000 NM³/hr from the Prill tower, but with the main Ammonium Nitrate dust load located near the prill head, extraction was split between the existing side outlets, fan vented to atmosphere and prill head shroud outlets.
A system would be housed in a vessel housed centrally on the top of the tower, above the prill head.
PROBLEMS TO SOLVE
The duty of the first supply was to handle 28,000 NM³/hr at 80° C. Dust load - 50 mg/NM³ minimum.Allowable pressure loss 300 to 350 mm. water column.NH3 load - 120 mg/NM³.The system would have to ensure continual saturation of the filters and to design in safety wash nozzles to prevent crystal build-up. The untreated gas to atmosphere contained about 200 Kg. per day of AN dust and the system was to treat the maximum AN dust with minimum gas flow.
The second supply was based on a re-design to reduce the total tower emission to less than 100 Kg/day of AN, but without extra filtration space.
DESIGN SOLUTION
A vessel with 4 side inlets at the bottom
- a first-stage BECOIL mesh pad in 304L stainless steel irrigated from above and below with 30% AN solution
- second stage 17 BECOFIL candle filters 607 mm. OD x 508 mm. ID x 3050 mm. long, standing type F, with PP12 Polypropylene fibre and 304L cages and flanges. The candle filters were fitted with a wash nozzle under the top flange to spray 0.8 litre/minute at 0.5 bar of demineralised water.
The mesh pad irrigation kept the temperature of the gas below 70° C to protect the polypropylene fibre, and, being AN solution, the NH3 is absorbed in the 310 litres/min. (at 1 bar).We guaranteed:
- Exit of 5 mg/NM³ of AN for an inlet of 50 mg.
- Exit of 5 mg/NM³ of NH3 for an inlelt of 120 mg.
Result: Less than 1 mg/NM³ AN. The scrubbing liquor was later changed to HNO3 solution at a controlled PH of 5. This gave no crystallisation on the candles and no need to spray wash.
For the revised duty, we considered that Begg, Cousland B12 fibre would give exit of 3 mg/NM³ of AN, so it would be possible to handle 120,000 NM³/hr through 17 filters packed with B12 fibre elements, the pressure loss would remain at about 350 mm. H2O, and the exit would be only approximately 9 Kg/day (i.e. 50% of the tower's maximum gas flow via the shroud exits).
This meant that the untreated gas exit would be only 120,000 NM³/hr., and the total exit would be close to the 100 Kg/day figure.
We guaranteed:
- Exit of 3 mg/NM³ AN maximum.
- Removal efficiency 100% * 3 micron 95% 1 to 3 micron & 80% 0.5 to 1 micron
Result: Operating to design and guarantees.
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