WSA-DC Sulfuric Acid Plant – Comprehensive Study Notes

Introduction to the WSA-DC Plant

• Purpose: Treat hazardous refinery waste gases (acid gas from ARU + sour gas from SWS) and convert H2S → H2SO4 (98.5 wt.%) • Dual benefits  ◦ Environmental – eliminates toxic H2S, SO2, NOx, acid mist
 ◦ Economic – produces marketable sulfuric acid + export steam
• Overall route (eight principal stages)
 1. Combustion
 2. NOx removal (SCR)  3. 1st SO2 oxidation + cooling
 4. 1st acid condensation
 5. Re-heat
 6. 2nd SO_2 oxidation + cooling
 7. 2nd acid condensation
 8. Mist control + WESP + product acid cooling
• Strong heat-integration: waste-heat boiler, two steam drums, steam super-heating, air loops, boiler-feed-water (BFW) pre-heaters.

Major Equipment & Primary Functions (condensed)

• Combustor (205-FB-01) – incinerates feeds → hot SO2 gas • K.O. drums – remove entrained liquids; safeguard combustion • Waste-Heat Boiler (205-E-01) – cools gas, generates saturated steam • Steam Super-heater – raises steam to export conditions • SCR reactors – reduce NOx with NH3 from SWS gas • 1st SO2 converter (2 adiabatic beds) – oxidise SO2 • Inter-bed cooler – steam generation + temperature trim • 1st / 2nd process-gas coolers – additional cooling + saturated steam • 1st WSA condenser (205-E-06) – cool, hydrate SO3, condense acid
• Acid concentrator – raise acid to 98.5 wt.% using dry air @ 275^{\circ}C
• Process-gas blower – overcome plant ΔP
• Process-gas heater – gas/gas exchanger with SO3-rich gas • Support heater – duct burner for start-up / turndown heat • 2nd SO2 converter – final oxidation
• 2nd WSA condenser – second acid capture
• Mist-control units – silicone nuclei generation
• WESP – electrostatic removal of residual mist
• 1st/2nd acid vessels & coolers – mix hot & cold acid; product draw-off or recycle
• 1st/2nd steam drums – separate steam; circulate boiler water
• 1st/2nd BFW pre-heaters – recover air & condensing-steam heat.

Feed Preparation & Combustion

• Feed sources:
 ◦ Acid gas (ARU)
 ◦ Sour gas (SWS)
• Liquid knockout: LIC-1601 / 1604 maintain drum levels; SRV → acid flare
• Heat-traced lines ⇒ avoid condensation
• Combustion conditions:
 ◦ T = 1300^{\circ}C
 ◦ Plant air + recycled dilute acid atomised
 ◦ 8 wt % O_2 in combustor exit gas (controls stoichiometry & downstream oxidation)
• Fuel-gas usage: normally 0; added for start-up, turndown, heating
• Hot combustion air source: preheated by 1st WSA condenser exhaust air; supplied via 1st combustion-air blowers (2×100 % duty, run 50 % each).
• Immediate cooling: Waste-heat boiler (bypass temperature control) → Steam Super-heater (conditions export steam) → SCR.

NO_x Reduction System (SCR)

• NOx formation in combustor via thermal mechanism at 1300^{\circ}C. • Ammonia source: SWS gas (contains NH3).
• Mixing steps:
 ◦ Recycle process gas (heated in 1st PG cooler) + SWS gas (heated in hot-air heater) ⇨ safe, below LEL, avoid ammonium-bisulfate fouling.
• Reaction on DNX catalyst:
 4\,NO{(g)} + 4\,NH{3(g)} + O{2(g)} \;\rightleftharpoons\; 4\,N{2(g)} + 6\,H2O{(g)} \;+\;97\;\text{kcal·mol}^{-1}
• Key variable: NH3/NOx < 1.0 ⇒ proportional removal.

1st Stage SO_2 Conversion & Cooling

• 1st SO2 converter (two adiabatic beds, VK catalyst). Reaction:  SO{2(g)} + \frac12 O{2(g)} \;\rightleftharpoons\; SO{3(g)} \;+\;24\;\text{kcal·mol}^{-1}
• Inter-bed cooler brings gas to optimal second-bed temperature via steam generation.
• Downstream PG coolers:
 ◦ 1st PG cooler (with PG heater shell-side) – adjustable
 ◦ 2nd PG cooler – boiler generating saturated steam
• Hydration begins:
 SO{3(g)} + H2O{(g)} \to H2SO_{4(g)} \;+\;25\;\text{kcal·mol}^{-1}
• Temperature window: above acid-dew-point, below 1st WSA condenser mechanical limit; prevents glass-tube corrosion.

1st Stage Acid Condensation & Concentration

• 1st WSA condenser: vertical glass falling-film tubes; simultaneous
 1. Gas cooling
 2. SO3 hydration  3. H2SO_4 condensation.
• Cooling medium: filtered ambient air via 1st cooling-air blower; hot exhaust reused.
• Acid concentrator (integral bottom): hot dry air (275^{\circ}C) contacts acid → 98.5 wt % product; dry-air unit filters + dehumidifies.
• Hot-air utilisation:
 ◦ Combustor combustion air
 ◦ Mix with 2nd WSA condenser air → 1st BFW pre-heater
 ◦ Cold-weather conditioning of condenser air.

Process-Gas Re-Heating & 2nd SO_2 Conversion

• Need to raise gas T > dew-point to avoid PG blower / heater corrosion.
• Steps:
 1. Mix with support-heater gas in PG mixers
 2. PG blower (overcome total ΔP)
 3. PG heater (gas–gas) – shell: cool PG; tubes: hot SO3 gas from 2nd converter  4. Additional heat from 1st PG cooler shell-side  5. Support heater (duct burner) if internal heat inadequate (start-up/turndown) • 2nd SO2 converter: final high conversion; same VK catalyst; equilibrium favoured (low SO_2 level).
• Converter exit → PG heater (cools to below 2nd WSA condenser design T).

2nd Stage Acid Condensation

• 2nd WSA condenser identical in design/function; cooling air via 2nd blower; captures residual acid.
• Reinforces emission compliance + product recovery.

Acid Mist Control & Minimisation

• Mist-control units (upstream of both condensers)
 ◦ Burn silicone-oil vapour → sub-µm SiO_2 particles → act as nuclei → grow droplets large enough to settle in condensers.
• WESP (wet electrostatic precipitator)
 ◦ Upper section: electrostatic field charges/collects remaining droplets.
 ◦ Lower section: quench zone; dilute acid bleed returns to 2nd acid vessel recycle.
 ◦ Clean gas + hot air from 2nd WSA condenser → stack.

Sulfuric Acid Cooling & Product Handling

• 1st acid loop
 ◦ Hot acid from concentrator + cold recirc in 1st vessel
 ◦ 1st acid cooler (CW) lowers T
 ◦ Majority recirc; slipstream → product (battery-limit).
• 2nd acid loop
 ◦ Similar mixing/cooling
 ◦ Product split: major recirc; minor dilute acid recycled to combustor (internal sulphur loop).
• Off-spec acid regeneration: optional, but fuel-intensive due to low heating value.

Integrated Heat-Exchange & Steam System

• BFW pump splits flow:
 1. Stream-1 → 1st BFW pre-heater (hot air shell) → 2nd BFW pre-heater (condensing steam) → 1st steam drum
 2. Stream-2 → 2nd steam drum (coil pre-heat prevents condensation in down-comers).
• Steam generation:
 ◦ Waste-Heat Boiler ↔ 1st steam drum
 ◦ 2nd PG cooler ↔ 2nd steam drum
 ◦ Inter-bed cooler + Steam super-heater raise export steam T.
• Condensate/blow-down:
 ◦ Continuous blow-downs to blow-down drum → flash LP steam (export) + cooled condensate (battery-limit).
 ◦ Dry-air heater condensate → LP flash → LP steam (export) + LP condensate (to WESP).
• Energy synergy: minimises external utilities; improves carbon footprint.

Process Control Philosophy & Key Loops

• Feed flexibility: plant accepts all available ARU/SWS gas; can swap between air-only and feedstock in stepwise fashion.
• Control modes:
 ◦ Normal: pressure-control (PIC) – receive whatever gas; maintain combustor pressure.
 ◦ Start-up: flow-control (FIC) – smoother ramp-up.
• Feed K.O. drums: LIC-1601/1604 on/off; protect combustor
• Feed flow cascade: FIC-1701/1702 cascaded to PIC-1701/1702; valves FV-1701/1702.
• Fuel-gas FIC-1705; set-point from high-selector FY-1714 (combines different demand signals).
• Combustion-air flow FIC-1716; drives FV-1716; redundant blowers K-03 A/B (each 100 % capacity).
• Temperature-critical loops:
 ◦ Combustor outlet, PG heater outlet, condenser inlets/outlets, steam super-heat.
• Safety features: SRVs to acid flare, interlocks on dew-point temperature, high SO_2 emission alarms.

Key Operational Insights & Design Philosophies

• Integrated heat recovery is central (WHB, PG coolers, air loops, steam system).
• Multi-stage SO2 oxidation + condensation maximises conversion and minimises emissions. • Pollution control comprehensive: internal NH3 for SCR; silicone-nuclei + WESP for acid mist.
• Materials & corrosion management: glass tubes, reheats, dew-point control, BFW design.
• Advanced control architecture: cascade, selectors, redundancy enable flexibility (start-up, turndown).
• Circular-economy practices: recycle dilute acid, reuse SWS gas, regenerate off-spec acid.

Quick Reference Tables

Key Process Parameters (selected)

• Combustor temperature ≈ 1300^{\circ}C (complete H2S burn). • O2 in combustor outlet = 8 wt %.
• 1st WSA condenser acid-dew-point target ≤ 263^{\circ}C.
• Product acid concentration = 98.5 wt %.
• Dry-air heater temperature = 275^{\circ}C.
• NH3/NOx (SCR) < 1.0 for proportional removal.
• PG temps:
 ◦ Post-cooler: well above dew-point & < 1st WSA mechanical limit
 ◦ Re-heat: above dew-point (protect blower/heater)
 ◦ 2nd SO_2 converter inlet: optimised for final conversion.

Principal Exothermic Reactions & Enthalpies

• H2S{(g)} + 1.5\,O{2(g)} \to SO{2(g)} + H2O{(g)} \;+\;91 kcal mol^{-1}
• Side formation: SO{2(g)} + H2O{(g)} \to H2SO{3(g)} (undesired) • SCR: see equation above (−97 kcal mol^{-1}). • SO2 oxidation: −24 kcal mol^{-1}.
• Hydration to H2SO4(g): −25 kcal mol^{-1}.