Pipe Fittings for Sulphuric Acid Alkylation Units: Grade Selection and Critical Boundaries
Sulphuric acid (H₂SO₄) alkylation units in refineries use 88–98% H₂SO₄ as the catalyst for isobutylene alkylation. The acid is handled at near-ambient temperature in a circulating acid-hydrocarbon emulsion system. While anhydrous or near-anhydrous concentrated H₂SO₄ is actually less corrosive to some materials than dilute acid, the alkylation unit introduces dilution, temperature transients, and organic contamination that make material selection more complex than simple acid concentration tables suggest.
Corrosion Mechanism in H₂SO₄ Alkylation
At concentrations above ~70% H₂SO₄, the acid is a relatively mild corrosive to carbon steel at ambient temperature — a sulphate film forms on the surface. Below ~70% (as the acid becomes spent and diluted), corrosion of carbon steel increases sharply. The acid temperature is also critical: above ~35°C, carbon steel corrosion rate in 88–96% H₂SO₄ increases rapidly. Most H₂SO₄ alkylation units operate the acid circuit at 5–15°C to maintain acid strength and minimise corrosion — the refrigeration system is therefore a critical component of the materials management strategy.
Material Selection by Location
| Location | Acid Condition | Fitting Grade |
|---|---|---|
| Reactor / contactor circuit | 88–96% H₂SO₄, 5–15°C | Carbon steel WPB |
| Acid settler / separator | 88–93% H₂SO₄, ambient | Carbon steel WPB |
| Spent acid return (diluted) | Below 88%, warming | Alloy 20 (N08020) or 904L |
| Acid loading / unloading arms | Variable concentration, ambient | Alloy 20 or 904L |
| Acid regeneration feed | 70–88%, elevated temp | Hastelloy C-276 or B-3 |
| Instrument connections throughout | All — copper prohibited | 316L SS — no brass |
Why Carbon Steel Works in the Main Circuit
The 88–96% H₂SO₄ at 5–15°C condition is one of the few environments where carbon steel WPB is genuinely the correct choice for acid service — the sulphate film that forms on steel surface in concentrated cold acid provides a degree of protection. Specifying 316L or Alloy 20 for the main circuit is not only unnecessary but counterproductive — austenitic stainless steel is attacked by concentrated cold H₂SO₄ because the passive Cr₂O₃ film dissolves, leaving the steel without protection. The material selection table reverses compared to most acid systems: carbon steel in the concentrated cold zone, alloy in the diluted or warmer zones.
Specifying for H₂SO₄ Alkylation Service
On the PO for the main circuit: WPB per ASTM A234, with a note "H₂SO₄ service — maximum operating temperature 20°C, acid concentration minimum 88%." The temperature and concentration limits belong on the PO so the manufacturer's review identifies any atypical requirements. For the spent acid and regeneration sections: Alloy 20 (ASTM B366 WPNC, UNS N08020) with EN 10204 3.1 certificate and chemistry confirming Nb ≥8×C for weld sensitisation protection.