Nitric acid production and concentration system

01

Choose the process scheme and product strength

EPA's AP-42 Section 8.8 explains that nearly all US nitric acid is made by high-temperature catalytic oxidation of ammonia in three steps. A 1:9 ammonia–air mixture is oxidised at 1,380–1,470 °F (up to 1,650 °F) over a catalyst, in AP-42's description, of 90% platinum and 10% rhodium gauze, giving nitric oxide in 93–98% yield; lower temperatures favour less useful nitrogen and nitrous oxide. After a waste heat boiler and platinum filter, the gas is cooled to 100 °F or less at up to 116 psia so that NO reacts with residual oxygen to NO₂, then absorbed counter-currently on sieve or bubble-cap trays with deionised water and secondary air, giving 55–65% acid in AP-42's description (range 30–70%) depending on temperature, pressure, stages and NOx concentration. Older plants run at one pressure, from atmospheric to 203 psia; because oxidation favours low pressure and absorption high pressure, newer dual-pressure plants oxidise at slightly negative to about 58 psia and absorb at 116–203 psia, bleach the acid with air, reheat the tail gas to about 392 °F and expand it in a power-recovery turbine that can drive the compressor. [1]

High-strength acid of 98–99% is made from weak acid by extractive distillation with a dehydrating agent, such as concentrated sulfuric acid: sulfuric acid and 55–65% nitric acid are fed to the top of a packed dehydrating column at about atmospheric pressure, 99% nitric acid vapour leaves the top to a bleacher and condenser, and recovered NOx returns to an absorption column as weak acid. [1]

Give the supplier the capacity (as 100% HNO₃ per day), product strength(s), single- or dual-pressure preference, ammonia and air supply conditions, catalyst and recovery-filter arrangement, steam export and power recovery targets, tail-gas NOx and N₂O limits and the control technology, concentration route and dehydrating agent, materials of construction, utilities, site and permitting constraints.

02

Control NOx in the absorber tail gas

AP-42 identifies the absorber tail gas as by far the largest NOx source; emissions rise with insufficient air, low absorber pressure, high cooler-condenser or absorber temperatures, excessively strong product, high throughput and faulty compressors or pumps. Extended absorption adds trays, raises absorber pressure, cools the weak acid or adds a tower; catalytic reduction heats the tail gas, mixes it with fuel such as natural gas or hydrogen and reduces NOx to nitrogen over a catalyst, can achieve greater reduction and allows heat recovery, though high fuel costs have reduced its use. Its emission factors (as 100% acid) are 57 lb NOx per ton uncontrolled from a weak-acid plant, 0.4 lb/ton with natural-gas catalytic reduction (99.1% control), 0.8 with hydrogen, 1.9 with single-stage extended absorption and 2.1 with dual-stage, and 10 lb/ton from a high-strength acid plant. Its worked basis notes that 500 tons per day of 55% acid counts as 275 tons per day of 100% acid. [1]

Editorial calculation: for 275 tons/day of 100% acid, uncontrolled tail gas would emit about 275 × 57 ≈ 15,700 lb NOx per day; with natural-gas catalytic reduction about 275 × 0.4 = 110 lb/day, and with single-stage extended absorption about 275 × 1.9 ≈ 520 lb/day. These AP-42 factors are dated averages rated E; design to the permit and supplier guarantees.

03

Ask for N₂O abatement and a measured N₂O emission factor

The 2006 IPCC Guidelines state that nitrous oxide (N₂O) forms as an unintended by-product of the high-temperature catalytic oxidation of ammonia, in amounts that depend on combustion pressure and temperature, catalyst composition and age, and burner design, and that nitric acid production is the major source of N₂O in the chemical industry. Their Table 3.3 gives default factors per tonne of 100% acid by plant type. [4]

IPCC groups abatement by position: primary measures in the ammonia burner, secondary measures removing N₂O from the process gas between converter and absorber, tertiary measures on the tail gas, best placed upstream of the expansion turbine, and quaternary measures after the expander. It calls direct tail-gas decomposition attractive above 723 K tail-gas temperature, and notes that process-gas decomposition in the burner can be applied to existing plants. [4]

Editorial calculation: for the 275 t/day (100% acid) example above, 9 kg/t gives about 2,475 kg N₂O a day for a high-pressure plant and 2.5 kg/t about 690 kg a day with N₂O destruction; 723 K is about 450 °C. Ask for a guaranteed, measured N₂O value rather than these defaults. [4] [1]

State the N₂O limit or target, the abatement position (burner, process gas or tail gas) and the measurement method for the guarantee.

04

Pick the pressure level against yield and tail-gas NOx

Fertilizers Europe's BAT booklet No. 2, Production of nitric acid (2000 edition), classifies plants by pressure: low (below 1.7 bar), medium (1.7–6.5 bar) and high (6.5–13 bar), and covers weak acid of 50–65% used by the fertilizer industry. It gives the NO yield of ammonia oxidation falling as pressure rises, and lists the gauze catalyst as about 90% platinum alloyed with rhodium, with a palladium alloy catchment recovering 60–80% of the platinum lost. [3]

The booklet notes that NSCR units need tail gas preheated to at least 300 °C with hydrogen or 550 °C with methane and also reduce N₂O, that an SCR unit taking tail gas from 1,000 to 200 ppmv NOx adds about 1.1% to operating cost with maintenance cost of about 2.5% of the SCR capital cost, and that start-up and shut-down raise stack NOx for a few hours. [3]

Editorial calculation: at 3,300 Nm³ of tail gas per tonne of acid, a 1,000 t/d plant (100% HNO₃ basis) releases about 3.3 million Nm³ of tail gas a day.

State the plant capacity (t/d of 100% HNO₃), product strength, the absorber pressure scheme and the NOx and N₂O limits the plant must meet, and ask for guaranteed tail-gas values and abatement method.

05

Compare plant configurations

No manufacturer record has been reviewed for this product yet; the configurations below come from the source cited.

Configurations include single-pressure and dual-pressure weak acid plants, with tail-gas control by extended absorption, catalytic reduction or chilled absorption with caustic scrubbing, and high-strength concentration units using extractive distillation; EPA's background report for the section gives the same process basis, including the 90% platinum / 10% rhodium catalyst. [1] [2]

06

Trace the plant units

This is a functional reference built from the sources cited; it is not one maker's specification.

07

Fill the specification fields with values for your plant

A nitric acid plant quotation needs values for your plant. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

08

Send the capacity, strength and emissions data together

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

Capacity: [t/day 100% HNO₃]
Products: [weak %; strong %]
Scheme: [single or dual pressure]
Energy: [steam export; power recovery]
Emissions: [NOx; N₂O; control method]
Concentration: [route; agent]
Site: [utilities; materials; permits]

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