Multi-stage vacuum evaporator train for ammonium nitrate: the flash vessel, vacuum falling-film stages and final concentrator that take neutraliser solution up to melt strength

01

From 59.9 % to 98 % in four steps: stage pressures, temperatures and flows in a 54 % nitric acid plant

US 4,645,656 (Pouilliart and Van Hecke, priority 1983) gives a full stage-by-stage balance for a plant that makes 98 % ammonium nitrate solution from 54 % nitric acid without importing steam. The solution leaves the reaction loop at 180 °C and passes through four concentration steps, each at its own pressure. [1]

Editorial calculation from the table: the train removes 41.77 − 25.5 = 16.27 t/h of water. The first depressurisation with evaporator 13 takes out 9.27 t/h, reboiler 39 2.2 t/h, reboiler 42 3.7 t/h and the final boiler 1.1 t/h. The ammonium nitrate itself stays at about 25.0 t/h through the train (41.77 × 0.5986 and 25.5 × 0.980), which is a check that the stated flows and concentrations agree. [1]

The same patent states the energy put into the reboilers: 4.75 GJ/h to the final boiler 3 and 10.47 GJ/h to reboiler 39, with 5.68 GJ/h to recuperator 20. In the patent's figures each vacuum stage has its own vacuum pump, cooler-condenser and barometric guard, and the temperature difference between the solution leaving evaporator 13 and the condensate of its heating steam is about 4 °C. [1]

The point of splitting the train is shown in the patent's comparison with a single flash to the same final pressure. With 70.2 % feed, a single flash to 0.2 bar gives 78.7 % and evaporates 154 kg of water per tonne of ammonium nitrate; flashing first to 0.7 bar and using that flash steam to heat a second evaporator at 0.2 bar gives 85.6 % and 256 kg per tonne. With 56.0 % feed the figures are 66.1 % and 273 kg for the single flash against 77.5 % and 495 kg for the two-step route at 0.25 and 0.085 bar. [1] Editorial calculation: 256/154 is about 1.66 and 495/273 about 1.81, so the two-step route evaporates roughly two-thirds to four-fifths more water from the same heat of reaction.

Ask the train supplier for a stage table in this form: pressure, outlet temperature, flow and concentration at each step, the heating medium for each step, and the water removed by each.

02

Pressurised to 85 %, vacuum to 98 %, hot-air stripping above: a two-section vacuum falling-film pre-evaporator and an atmospheric finisher

Taiyuan Hailifeng's utility model CN 201962082 U (2010) assigns each concentration range its own operating condition: 65–85 % solution can be concentrated under pressure, 90–98 % under vacuum, and above 98 % atmospheric gas stripping is preferred. It builds the train on those bands and splits the output into a 90–97.5 % range and 99.7 %. [2]

The patent's starting point is a single-section evaporator heated only by neutraliser vapour. At 60 % feed acid and 4.5 bar (abs) the saturation temperature of that vapour is only 148 °C, and the solution reaches only about 95 %; the shortfall is made up with fresh low-pressure steam or by replacing the process steam altogether. [2]

In the Hailifeng train, 75–78 % solution from a pressurised neutraliser first enters a vacuum flasher fitted with a scum remover and leaves at 80–82 %. A vertical pre-evaporator then concentrates it to 96.5–97.5 % in two falling-film sections stacked in series: the upper section is heated by process steam and is about three times the length of the lower section, which is heated by fresh low-pressure steam. When only 90–92 % solution is needed for crystalline ammonium nitrate, the lower section runs without fresh steam. [2]

A liquid collector 450 mm high and 600 mm in diameter, about 1,200 mm from the lower tube sheet of the upper section, sits between the two sections. The pre-evaporator's distributor is a butterfly-dish assembly with a 1,090 mm base plate set 45 mm above the tube sheet. [2]

The final evaporator is a falling-film unit run at atmospheric pressure with hot-air stripping, heated by medium-pressure steam. It has 206 heat transfer tubes about 4,500 mm long on an equilateral triangular pitch, three baffles about 1,120 mm apart, and a hot-air distribution pipe with 420 holes of 25 mm. Its liquid distributor is a perforated plate with 203 holes of 6.5 mm and 15 holes of 5.5 mm, and a 38 mm vapour pipe for each tube. Stripping gas and non-condensables from the train are washed in a Venturi scrubber with evaporator condensate before discharge through an induced-draft fan. [2]

State the product the melt is for (crystalline, porous prills, calcium ammonium nitrate or high-density), because that fixes whether the train stops at 90–92 %, 96.5–97.5 % or needs the stripping finisher for 99.7 %.

03

One flash or a train: single-flash concentration to 90–96 % and a flash-plus-vacuum pair reaching 98 %

Two Chinese patents show where the line between a single flash and a multi-stage train falls. Sedin Engineering's CN 106365177 A presents its single-step vacuum flash as a way to avoid multi-stage concentration altogether, while Sichuan Golden Elephant's CN 112028089 B adds one vacuum evaporator after the flash to reach 98 %. [3] [4]

In Sedin's process, gaseous ammonia and nitric acid react in a tubular reactor at 0.3–0.5 MPa and 190–195 °C; the 75–80 % solution passes through a Venturi throttle into a flash tank whose vapour space is held at 0.03–0.08 MPa, and leaves at 90–96 % using only the heat of reaction. In one example, 56 % acid reacted at 0.35 MPa and 190 °C, the flash tank ran at 0.08 MPa with the liquid at 130 °C, and the product was 90 %. [3]

Golden Elephant's plant flashes 80 % solution from 0.4 MPa to atmospheric pressure, giving 92 % with flash steam at 150 °C, then concentrates it to 98 % in an evaporator held at −0.06 MPa. The patent's preferred evaporator range is −0.09 to −0.01 MPa, and it states the vacuum evaporator may be built as a multistage unit. Its heating steam comes from a vapour compressor that raises low-pressure steam from the process condenser to 0.4 MPa, 2 t/h of which heats the evaporator in the example. [4] Editorial calculation: −0.06 MPa gauge is about 0.04 MPa absolute, taking atmospheric pressure as 0.101 MPa.

Read together, the four patents put the single flash at 90–96 % [3] and the flash with one vacuum evaporator at 98 % [4], while 98 % from 54 % acid without imported heat takes four steps [1] and 99.7 % takes a stripping finisher after the vacuum stages [2].

If a single flash is offered instead of a train, ask for the flash pressure, the outlet temperature and the concentration reached with the plant's own nitric acid strength.

04

Data for an ammonium nitrate evaporator train

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Feed: [AN concentration and temperature from the neutraliser; flow; neutraliser pressure; nitric acid strength]
Product: [final concentration; end use — crystalline, porous prills, CAN, high-density]
Stages: [number; type — flash, falling-film, laminar-film, reboiler, stripping finisher; pressure and outlet temperature of each]
Heating: [process vapour, low-pressure or medium-pressure steam, vapour compression, for each stage]
Vacuum: [pump or ejector per stage; condenser; barometric leg]
Off-gas: [scrubber type; discharge limit]
Transfer: [inter-stage tanks and pumps; ammonia trim between stages]

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