1. Where sour water comes from
"Sour water" is process water that has picked up dissolved H₂S and NH₃ from contact with gas or oil — overhead condensers on amine regenerators, crude desalter wash water, FCC fractionator condensers, and hydrotreater stripper overheads are the classic sources. The combined SWS feed in a refinery is typically 50–500 gpm at 1000–5000 ppm H₂S and 500–3000 ppm NH₃.
The stripper sends the recovered H₂S/NH₃ acid gas to a Claus unit or thermal oxidizer and produces a stripped-water bottoms suitable for reuse as desalter wash, boiler feed (after polishing), or sewer discharge depending on the spec.
2. The Kremser equation
For a column stripping a volatile solute from a non-volatile liquid using a clean (y_in = 0) stripping gas, the Kremser equation gives the theoretical-stage count:
where the stripping factor is S = K·V/L (V = stripping-medium molar rate, L = liquid molar rate). The result is the number of equilibrium stages — multiply by an overall tray efficiency η ≈ 0.55 (GPSA SWS recommendation) to get actual sieve trays. Typical SWS designs land at 6–10 actual trays.
S > 1 is required to strip below the inlet concentration; S < 1.2 is "pinched" — small spec tightening cause N to blow up. Most operators target S = 2–3 for NH₃.
3. K-values: why NH₃ controls
| Species | K (at 230 °F, 25 psig) | S at Rsteam=1.2 lb/gal | Implication |
|---|---|---|---|
| H₂S | ~200 | ~240 | Strips off the top tray; only ~1 stage needed. |
| NH₃ | ~4 | ~4.8 | Requires 2–4 theoretical stages → 4–8 actual trays. Controls design. |
| CO₂ | ~500 | ~600 | Strips with H₂S; ignored in single-stage SWS. |
K-values are functions of T, P, and pH. In a real SWS, NH₃ in solution exists partly as NH₄⁺ + HS⁻ — the dissolved ion pair is much harder to volatilize than free NH₃. K_NH₃ values used here are "apparent" K values that lump the chemistry into a single number, valid for typical refinery SWS pH 8–10. For high-pH or caustic-injected feeds, K_NH₃ drops further and additional stages are required.
4. Steam ratio & reboiler duty
Typical stripping steam ratios are 1.0–1.5 lb steam / gallon sour water. At the lower end, NH₃ slip increases — at the higher end, reboiler duty (and operating cost) climbs without much marginal removal benefit. The reboiler duty:
The 1.1 factor accounts for heat losses, feed preheat shortfall, and a small margin. h_fg of 945 Btu/lb corresponds to ~30 psia (~250 °F) low-pressure saturated steam; adjust for your steam header. A 100 gpm column at R = 1.2 typically demands 50–70 MMBtu/hr reboiler — sized as a kettle or thermosyphon with a stainless 316L tube bundle (NH₃/H₂S/water is corrosive — specify metallurgy for ammonium bisulfide service).
5. References
- GPSA Engineering Data Book, §19 — Sour-Water Stripping.
- GPA Sour Water Equilibria (GPSWAT) — H₂S / NH₃ / CO₂ vapor-liquid equilibria for sour water systems.
- Edmister, W.C. & Maxwell — absorption/stripping-factor (Kremser) method, as applied in GPSA §19.
- Beychok, M.R. (1967). Aqueous Wastes from Petroleum and Petrochemical Plants. Wiley.
- Kremser, A. (1930). "Theoretical analysis of absorption process." National Petroleum News 22(21), 42–49.
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