Refinery Gas Treating, Sulfur Recovery Units (SRU) & Emissions
Claus Sulfur Recovery Unit (SRU) 1/3 Air Split & Yield Formula
Calculate Exact One-Third H2S Combustion O2 Demand (kmol/hr), Reaction-Furnace Dry Air Rate (kmol/hr), Recovered Liquid Elemental Sulfur (t/d), and Tail-Gas Incinerator SO2 (t/d).
Burn ONLY ONE-THIRD of acid-gas H2S in the Claus furnace—burning all H2S makes SO2 instead of elemental sulfur!
Amine regenerator acid gas from refinery hydrotreaters and sour natural gas sweetening contains concentrated H2S that must be converted into safe molten elemental sulfur S via the two-step Modified Claus Process: (1) Thermal Reaction Furnace burns EXACTLY 1/3 of incoming H2S with air (H2S + 1.5 O2 -> SO2 + H2O, consuming (1/3) * 1.5 = 0.50 kmol O2 per kmol of TOTAL incoming H2S!), leaving the remaining 2/3 unburned H2S at the stoichiometric 2 : 1 H2S-to-SO2 ratio needed for (2) Catalytic Alumina Beds: 2H2S + SO2 -> 3S + 2H2O! Feeding full combustion air (1.5 O2 per H2S) destroys Claus stoichiometry! Furthermore, while a 3-bed Claus unit recovers ~97.0% of sulfur, adding an amine SCOT Tail-Gas Treating Unit (TGTU) lifts recovery to 99.8%+!
Verify on a 500.0 kmol/hr Refinery Acid-Gas Feed (B2) with H2S_MolFrac = 0.80 (400.0 kmol/hr H2S, inert CO2 balance, B3) and 3-Stage Claus_Recovery = 0.970 (97.0%, B4): Exact Stoichiometric O2 for 1/3 H2S Oxidation is 0.50 * 400 = 200.00 kmol/hr (E2), Dry Air Blower Flow (at 21% O2) equals 200 / 0.21 = 952.38 kmol/hr (E3), Liquid Elemental Sulfur Recovered (MW = 32.06) hits (400 * 0.97 * 32.06 * 24) / 1,000 = 298.54 t/d (E4), and Tail-Gas Incinerator SO2 (MW = 64.06) equals 18.45 t/d (E5)!
| Claus SRU KPI (500 kmol/hr Acid Gas, 80% H2S, 97.0% 3-Bed Recovery) | Formula | Verified Output |
|---|---|---|
| 1/3-Burn Stoich O2 (kmol/hr) | (Acid_Gas * y_H2S / 3) * 1.5 = 0.5 * H2S | 200.00 kmol/hr |
| Furnace Dry Air Rate (kmol/hr) | Stoich_O2_kmol_hr / 0.21 | 952.38 kmol/hr |
| Recovered Molten Sulfur (t/d) | (H2S_kmol_hr * Rec * 32.06 * 24) / 1000 | 298.54 t/d (307.16 @ 99.8% SCOT) |
| Tail-Gas Incinerator SO2 (t/d) | (H2S_kmol_hr * (1-Rec) * 64.06 * 24)/1000 | 18.45 t/d (1.23 t/d @ 99.8% SCOT!) |
Modified Claus 2:1 Stoichiometric Air & Molten Sulfur Tonnage Formula
Validate Acid_Gas_kmol_hr > 0, H2S_MolFrac in [0.15, 1.0] (leaner gases need split-flow or oxygen enrichment), and Sulfur_Recovery in [0.90, 0.9999]!
=LET(ag_f, B2, yh_s, B3, sr_c, B4, IF(OR(ag_f <= 0, yh_s < 0.15, yh_s > 1, sr_c < 0.9, sr_c > 0.9999), "Invalid Claus SRU Input", LET(h2_s, ag_f * yh_s, ROUND((h2_s * sr_c * 32.06 * 24) / 1000, 2))))SCOT Tail-Gas Treating Unit (TGTU) 99.8% Upgrade Check (Set Recovery = 0.998)
Add a hydrogenation-amine SCOT Tail-Gas Treating Unit lifting Sulfur_Recovery B4 from 0.970 to 0.998: Molten Sulfur E4 rises to 307.16 t/d while Tail-Gas Incinerator SO2 Emissions E5 plummet 15x from 18.45 t/d down to just 1.23 t/d!
=IF(ISTEXT(E4), E4, ROUND((B2 * B3 * (1 - B4) * 64.06 * 24) / 1000, 2))Try the example
Use the original values or change them to test your understanding.