DAP granulation loop: pre-neutralizer, ammoniating granulator, scrubbers

Diammonium phosphate at the scale of one Jorf Lasfar (JESA) granulation train: 40.2 kg/s (145 t/h) of wet-process phosphoric acid is neutralised by ammonia in two steps and granulated in a loop that recycles four times its own product. The acid carries the composition Handley (IFA, 1984) gives for a commercial merchant-grade acid, per 100 g: 54.82 g H3PO4 (39.7% P2O5), 3.90 g H2SO4, 5.68 g of metal and fluoride impurities (carried as one inert lump) and 35.60 g water. It is pumped first through the tail-gas scrubber and the pre-scrubber, which take back the ammonia the reactors do not absorb, then into the pre-neutralizer, which holds a mole ratio NH3:H3PO4 of 1.44 at 114 °C (near the ratio of highest ammonium phosphate solubility, so the least water keeps the slurry fluid) and boils off water with its own reaction heat. The slurry is sprayed onto the rolling bed of the ammoniating granulator, where sparged ammonia lifts the ratio to 1.845. New solid layers onto the recycled granules (an equal-thickness growth law on the full size distribution), the bed is dried to 3% moisture, a double-deck screen splits it at 3.35 and 1.70 mm, the oversize is crushed, and fines, crushed oversize and 73% of the on-size go back to the granulator. The rest is cooled and sold. The 73% is the split that puts the loop at the recycle ratio Handley reports, 4: a plant sets its recycle to absorb the slurry's water, so here it is an input, not a prediction.

Solved: 33.7 kg/s of product (121 t/h), 18.7% N and 47.6% P2O5 on a dry basis, above the 18-46-0 commercial grade, which is set by the acid's sulfate and impurities rather than by the 21.2-53.8 of pure DAP. Consumption per tonne of product is 0.227 t NH3 and 0.474 t P2O5. An 18-46-0 product carries 0.219 t of NH3 as nitrogen and 0.46 t of P2O5 per tonne by its grade alone; the difference is this product's richer grade and the ammonia lost at the stack. The recycle ratio is 4.0, and the loop converges on its own tear machinery (six tears, Wegstein). The pre-neutralizer releases 19.1 MW of reaction heat and evaporates 7.3 kg/s of water; the granulator releases 6.9 MW and evaporates 5.2 kg/s; the dryer takes 1.7 kg/s more for 7.1 MW. 0.029 kg/s of ammonia leaves the stack, 0.4% of what is fed, which is what two scrubbers at 90% recovery each give.

Honesty notes: (1) The speciation is stoichiometric: MAP, DAP and ammonium sulfate are assigned from element totals, so the grade is exact for the stated acid while solution equilibria, heat of solution and heat of crystallisation are not in the balance. (2) The slurry water (18%) and the ammonia slip of each reactor (0.29 and 0.515 of the ammonia fed) are measurements from Handley's IFDC pilot plant, not predictions; the scrubber recoveries (0.9, after SPIC's report to IFA in 1998 that its venturi stages do about 90% of the scrubbing) and liquor mole ratios (1.5 and 0.7, JESA's stated practice) are inputs. (3) The crusher's breakage function (Austin, phi 1, gamma 5, beta 3) is illustrative of a chain mill, not fitted to one; it is what lets the loop settle with a product d50 of 2.3 mm. (4) The product cooler is a fluidised bed, one of the two cooler types the EFMA's BAT Booklet No. 8 (2000) names, swept by 500 mol/s of ambient air and fitted with a 30 °C cooling-water bank of 220 m2 at U = 250 W/m2/K. The area is sized so the product leaves at 48 °C, under the 120 °F (49 °C) TVA reports cooling ammoniation-granulation product to (OSTI 7013379); U, the air rate and the bed size are representative, not a plant's. (5) The two ammonia feeds are supply headers: what the reactors do not draw leaves through each one's excess-ammonia port and is not consumption. (6) The acid is fed as a stream at 47 °C; the phosphoric-acid plant upstream is not part of this flowsheet.