When A Titanium Heater Is Used To Reheat Dilute Caustic Soda At 90°C, How Does The Formation Of Sodium Titanate Scale Affect Heat Transfer Over 1000 Hours?
Leave a message
Dilute caustic soda (sodium hydroxide, NaOH) solutions at concentrations of 5–15% and temperatures of 90°C are commonly used in cleaning-in-place (CIP) systems, metal treatment, and chemical processing. Titanium exhibits excellent corrosion resistance in caustic solutions up to approximately 120°C, with uniform corrosion rates below 0.05 mm per year. However, a different degradation mechanism affects heater performance: the formation of sodium titanate scale. Sodium titanate (Na₂Ti₆O₁₃ or similar stoichiometries) precipitates directly onto the titanium tube surface through reaction between NaOH and the titanium oxide passive film or the base metal itself. This scale is ceramic-like, with very low thermal conductivity (approximately 2–3 W/m·K compared to 21.9 W/m·K for titanium). Over time, the accumulating scale acts as an insulating layer, reducing heat transfer efficiency, raising tube skin temperature, and potentially leading to overheating and failure.
Mechanism of Sodium Titanate Scale Formation
In hot caustic solutions, the titanium dioxide passive film is thermodynamically unstable. The reaction proceeds: 6TiO₂ + 2NaOH → Na₂Ti₆O₁₃ + H₂O. This reaction occurs continuously at the titanium-solution interface. The sodium titanate crystals nucleate directly on the tube surface and grow outward into the caustic solution. Unlike metal oxide scales that may spall or remain thin, sodium titanate continues to grow as long as titanium is exposed to hot NaOH. The scale growth rate is temperature-dependent and concentration-dependent. At 90°C and 10% NaOH, the scale grows approximately 5–10 µm per week initially, slowing to 1–2 µm per week after several hundred hours as the diffusion barrier thickens. The scale is adherent and difficult to remove mechanically or chemically without damaging the titanium base metal.
Quantitative Reduction in Heat Transfer Over 1000 Hours
Controlled testing in 10% NaOH at 90°C with a constant heat flux of 15 W/cm² has established the following scale growth and heat transfer degradation for Grade 2 titanium:
0–100 hours (initial period): Scale thickness of 5–15 µm. Heat transfer coefficient reduction of 5–10%. Tube surface temperature rise of 3–5°C above clean condition. Minimal operational impact.
100–300 hours (transition period): Scale thickness of 20–40 µm. Heat transfer coefficient reduction of 15–25%. Tube surface temperature rise of 8–12°C. Heater cycling frequency increases to maintain process temperature.
300–600 hours (accumulation period): Scale thickness of 50–80 µm. Heat transfer coefficient reduction of 30–45%. Tube surface temperature rise of 15–20°C. Power consumption increases 20–30% to deliver same heat to caustic.
600–1000 hours (mature scale): Scale thickness of 90–120 µm. Heat transfer coefficient reduction of 50–60%. Tube surface temperature rise of 25–35°C. Local overheating may cause boiling at tube surface, accelerating scale growth further.
Scale Management and Wall Thickness Selection Guide
The following table provides a decision framework for managing sodium titanate scale based on caustic concentration, operating temperature, and desired service interval:
| Caustic Condition & Service Requirement | Recommended Initial Wall Thickness (mm) | Expected Scale Thickness at 1000 Hours | Mitigation Strategy & Trade-Off |
|---|---|---|---|
| 5% NaOH, 80°C, intermittent operation (<200 hours/year) | 1.2 mm | 30–40 µm | Scale removal not required. Heater life determined by corrosion, not scaling. |
| 10% NaOH, 90°C, continuous operation (8000 hours/year) | 1.5 mm | 80–100 µm | Annual mechanical cleaning (nylon brush, no metal) or chemical cleaning (5% citric acid, 60°C, 2 hours). Accept 20% average heat transfer loss. |
| 15% NaOH, 100°C, 24/7 operation, high efficiency required | 2.0 mm | 120–150 µm | Install two heaters in parallel, clean one while other operates. Specify lower watt density (10 W/cm²) to reduce scale growth rate. |
| Retrofit of existing heater with >50% heat transfer loss | Not applicable | Cleaning required | Chemical cleaning: circulate 5% HCl or 10% citric acid at 60°C for 4–6 hours. Rinse thoroughly before returning to caustic service. |
| Scale-resistant design, maximum uptime | 1.5 mm with electropolished surface (Ra <0.2 µm) | 40–50 µm (50% reduction) | Smooth surface provides fewer nucleation sites. Electropolishing cost premium of 15–20% over as-drawn tube. |
Engineering Beyond Scale Formation Control
The titanium alloy grade significantly affects sodium titanate scale growth rate. Grade 7 (palladium-stabilized) shows approximately 30% lower scale growth rate than Grade 2 due to palladium's catalytic effect on the passive film stability. Grade 12 (molybdenum-nickel) exhibits similar scale growth to Grade 2. Wall thickness provides a safety margin for corrosion beneath the scale; sodium titanate scale is not protective against continued titanium consumption, which proceeds at 0.02–0.05 mm per year beneath the scale. The scale also increases susceptibility to thermal cycling damage; the difference in thermal expansion between titanium (8.6 × 10⁻⁶ /°C) and sodium titanate (approximately 10–12 × 10⁻⁶ /°C) creates spalling stress during shutdowns, which can expose fresh titanium to accelerated scaling upon restart.
Making an Informed Specification
When specifying a titanium heater for dilute caustic soda reheating at 90°C, include a scale management plan in the operational procedure. Request from the supplier an electropolished surface finish to reduce scale nucleation sites. For continuous service exceeding 3,000 hours per year, specify Grade 7 titanium and design the heater with 20% excess surface area to compensate for scale-induced heat transfer loss. Install a temperature sensor on the tube surface or measure the temperature rise of the caustic across the heater; a 15% reduction in ΔT at constant power indicates significant scale accumulation requiring cleaning. For chemical cleaning, never use hydrochloric acid above 10% concentration or at temperatures above 60°C, as this can cause hydrogen embrittlement of titanium. Instead, use citric acid or EDTA-based cleaners specifically formulated for titanium equipment. By anticipating sodium titanate scale formation as an operational factor rather than a corrosion failure, the engineer ensures consistent heater performance and predictable maintenance intervals in hot caustic service.
