Fertilizer & Industrial Chemical Manufacturing, Ostwald Nitric Acid
Ostwald Nitric Acid Pt-Rh Gauze Selectivity & Yield Formula
Calculate 100%-Basis Pure HNO3 Produced (t/d), Commercial Weak Nitric Acid Output (t/d), Effective Acid-to-Ammonia Mass Gain Ratio, and Unselective N2/N2O Nitrogen Loss (kmol N/d).
1 ton of NH3 theoretically yields 3.706 tons of pure HNO3—provided millisecond Pt-Rh gauze contact beats N2/N2O degradation!
In Ostwald nitric acid plants feeding ammonium nitrate and adipic/polyurethane intermediates, vaporized anhydrous NH3 (MW = 17.0 g/mol) mixed with compressed air passes in ~1 millisecond at 850–900°C across woven Platinum-Rhodium (90% Pt / 10% Rh) catalyst gauzes: 4NH3 + 5O2 -> 4NO + 6H2O, followed by cooler oxidation 2NO + O2 -> 2NO2 and pressurized absorption 3NO2 + H2O -> 2HNO3 + NO (MW = 63.0 g/mol)! Because NO is thermodynamically unstable relative to N2 and greenhouse-gas N2O at 900°C, Fouled or Rhodium-depleted gauzes lose Gauze NO Selectivity S_gauze (typically 95%–97%)! Multiplying by Absorber Recovery R_abs (~99.0%) gives 100%-Basis HNO3 = (NH3_t_d / 17) * S_gauze * R_abs * 63 and Commercial Acid Tonnage = Pure_HNO3 / Acid_Strength_WtFrac!
Verify on a 170.0 t/d Ammonia Dual-Pressure Nitric Acid Train (10,000 kmol/d NH3, B2) at Pt_Rh_Gauze_Selectivity S_gauze = 0.96 (96.0%, B3), Tower_Absorption_Recovery R_abs = 0.99 (99.0%, B4), and Commercial_Weak_Acid_Strength = 0.60 (60.0 wt% HNO3, B5): Pure 100% HNO3 Equivalent hits (170 / 17) * 0.96 * 0.99 * 63 = 598.75 t/d (E2), Commercial 60% Acid Output reaches 997.92 t/d (E3), Effective Pure-HNO3-to-NH3 Mass Ratio equals 3.522 (E4, vs 3.706 stoichiometric ceiling), and Gauze Side-Reaction Nitrogen Loss equals (170 / 17) * 1,000 * (1 - 0.96) = 400.00 kmol N/d (E5)!
| Ostwald Nitric Acid KPI (170 t/d NH3, 96% Gauze, 99% Abs, 60 wt% Acid) | Formula | Verified Output |
|---|---|---|
| 100%-Basis Pure HNO3 (t/d) | (NH3_t_d / 17) * S_gauze * R_abs * 63 | 598.75 t/d (573.80 @ 92% gauze) |
| Commercial 60 wt% Acid (t/d) | Pure_100Pct_HNO3_t_d / Acid_WtFrac | 997.92 t/d (956.34 @ 92% gauze) |
| Pure HNO3 / NH3 Mass Ratio | Pure_100Pct_HNO3_t_d / NH3_Feed_t_d | 3.522 (3.375 @ 92% gauze) |
| Gauze N2/N2O Loss (kmol N/d) | (NH3_t_d * 1000 / 17) * (1 - S_gauze) | 400.00 kmol/d (800.00 @ 92% gauze!) |
Ostwald Commercial Nitric Acid Output & Gauze Selectivity Formula
Validate NH3_Feed > 0, Gauze_Selectivity in [0.80, 0.995], Absorber_Recovery in [0.90, 0.9999], and Acid_WtFrac in [0.50, 1.0]!
=LET(nh_t, B2, sg_z, B3, ra_b, B4, ac_w, B5, IF(OR(nh_t <= 0, sg_z < 0.8, sg_z > 0.995, ra_b < 0.9, ra_b > 0.9999, ac_w < 0.5, ac_w > 1), "Invalid Ostwald Input", ROUND(((nh_t / 17) * sg_z * ra_b * 63) / ac_w, 2)))Aged End-of-Campaign Pt-Rh Gauze Selectivity Drop Check (Set Gauze Selectivity = 0.92)
Drop Gauze_Selectivity B3 from 0.96 to 0.92 (platinum volatilization / iron-oxide fouling at end of campaign): Commercial 60% Acid E3 loses 41.58 t/d (falling to 956.34 t/d) and Unselective N2/N2O Nitrogen Loss E5 doubles from 400.00 to 800.00 kmol N/d!
=IF(ISTEXT(E3), E3, ROUND(((B2 * 1000) / 17) * (1 - B3), 2))Try the example
Use the original values or change them to test your understanding.