Reactor Material Selection: High Temperature, Attrition and Corrosion
1. Temperature: Creep and Oxidation Set the Ceiling
Silicon-carbon coating runs at 450–550°C; carbonisation and activation reach 800–950°C. Two mechanisms decide survival — high-temperature strength and oxidation resistance.
Creep governs strength. 304H accelerates above 550°C, and at 600°C its allowable stress is about one fifth of room-temperature value. 316L reaches 550–600°C thanks to molybdenum, not 650°C. 310S resists oxidation to 1,000–1,050°C through 25% chromium and 20% nickel, yet loses strength fast above 600°C — design it 1.5–2 times thicker than a cold-wall vessel.
Oxidation sets the clock. Carbon steel scales at 0.1–0.3 mm per year above 500°C, halving an 8 mm wall in two years. 304 runs 0.02–0.05 mm/y at 600°C, 310S 0.01–0.03 at 800°C. Above 600°C with three-year continuous duty, stop arguing about 304.
Beds swing ±10°C normally, but wet feed and bridging push peaks 40–60°C above setpoint. Size for setpoint plus 50°C.
2. Attrition: Why the Distributor and Cyclone Fail First
Wear follows gas velocity, which is never uniform.
Distributor. Nozzle velocity is 20–40 m/s against a superficial 3–6 m/s — six to ten times higher. Solids remove 1–3 mm per year at nozzle exits, so a 12 mm 316L plate on hard powder wears through in two to three years.
Cyclone. Inlet velocity of 18–25 m/s concentrates wear on the leading third of the involute and the lower cone. Carbon steel units on silicon-carbon powder last 8–14 months; once perforated, fines escape to the off-gas and yield drops.
Shell walls wear gently, but reducers, weld beads and flanges triple local wear; grinding welds flush is a wear measure, not cosmetics.
Fixes: thick plates with replaceable 310S or ceramic nozzles, wear sections at cyclone inlets, flush-ground welds.
3. Corrosion: Hydrogen, Chlorides and Steam
Hydrogen. Above 400°C hydrogen reduces the oxide layer and diffuses in, causing decarburisation and embrittlement. Thickness barely changes while toughness falls; welds crack first. The answer is lower hydrogen partial pressure: dilute to 20–50% H2 in argon or inject separately.
Chlorides. Feed chlorine of 0.05–0.3% leaves as HCl at 50–500 ppm. Its dew point band of 200–400°C coincides with the upper shell, cyclone and off-gas duct, where condensate pits through in months. Above 50 ppm chloride, use 2205 duplex or line the section.
Steam above 850°C accelerates oxidation 1.5–2 times over air; activation linings rarely exceed two to three years.
4. Alloy, Clad Plate or Lining
Alloy. At 400–550°C, 316L or 304H at 8–12 mm, cost index 1. Above 600°C it means 310S at 2.2–2.8 times the cost, or Inconel 601 at 6–9 times.
Clad plate. Q345R backing plus 3–5 mm 316L or 310S, about 0.6–0.7 of solid alloy. Suits corrosion-dominated duty below 500°C. Once cladding wears through, the backing corrodes fast — keep it out of the bed and cyclone inlets.
Lining. An 80–150 mm castable drops wall temperature from 800°C to 150–250°C; cost rises 15–25%, half recovered by downgrading the shell. The trade is maintenance: 24–72 hours of staged dry-out, two to four years of life, patches every 6–12 months costing 3–7 days down. With one window a year, think twice.
Verify rather than assume: ultrasonic survey every 3–6 months at the distributor, cyclone inlet and reducers, replacement forced at 30% loss. Units designed for 8–10 years overhaul at 5–7.
Frequently Asked Questions (FAQ)
Q: Is 304 enough below 550°C? A: For dry, chloride-free duty, yes — but check thickness at 600°C. With 50 ppm HCl, expect one to two years.
Q: How often should the distributor be inspected? A: Every three months at nozzle exits and plate centre. A 12 mm plate is replaced at 30% loss.
Q: Does lining really cost less than alloy? A: First cost is 15–25% higher, though the shell can be downgraded. Downtime decides.
Q: Can a worn cyclone be welded? A: Only as an emergency fix; repair welds become the next failure point.
Keywords: reactor material, high temperature alloy, distributor plate, cyclone wear