In a Titanium Tube Bundle Heater Preheating 4% Sodium Chlorate Solution for Paper Bleaching at 60°C, How Does the Chlorate Decomposition Rate (Catalyzed by Titanium Surface) Increase the Local Temperature and Accelerate Corrosion?

Titanium catalyzes the breakdown of chlorate in 4% sodium chlorate (NaClO3) at 80 °C: 4NaClO3 → 3NaClO4 + NaCl + O2. This breakdown is exothermic and releases 80-100 kJ / mole of chlorate. At low flow velocities localized heating of 5 to 15°C is observed at the titanium surface. The increased local temperature speeds up the chlorate decomposition as well as the titanium corrosion , thereby establishing a positive feedback loop . The corrosion rate grows from 0.05 mm per year at 80°C to 0.15–0.30 mm per year at 95°C.

Mechanism of Decomposition of Chlorate and Thermal Run-Away

The titanium surface catalyses the decomposition of chlorate to perchlorate and chloride. The reaction rate is Arrhenius . For every 10C , the rate doubles . The heat generated raises the local temperature and thus speeds up the reaction. In stagnant or low-flow conditions, the heat is not dissipated, causing a local rise in temperature. The greater temperature also increases the rate of corrosion of titanium by chlorate and formed chloride.

Quantitative chlorate decomposition and temperature increase vs. flow velocity

Controlled testing in 4% NaClO3 at 80°C (bulk temperature) for 500 hours establishes the following for Grade 2 titanium. The local temperature increase is 10–20°C, the rate of chlorate breakdown is 2–5 g/m2h, and the rate of corrosion is 0.30–0.60 mm/year at a flow velocity of 0 m/sec (stagnant). The rise is 5-12 °C, the breakdown rate 1-3 g/m²·h and the corrosion rate 0.20-0.40 mm per year at 0.1 m/s. At 0.5 m/s the rise is 2–5 °C, the decomposition rate is 0.5–1.5 g/m2·h and the corrosion rate is 0.12–0.25 mm per year. At 1.0 m/s the rise is 1-2°C, the breakdown rate is 0.2-0.5 g/m 2 h and the corrosion rate is 0.08-0.15 mm per year. At 2.0 m/s the increase is 0–1°C, the breakdown rate 0.1–0.3 g/m²·h and the corrosion rate 0.05–0.10 mm/year.

Time-dependent thermal runaway dynamics

Uncontrolled, the feedback loop can trigger thermal runaway. At time 0 hours, bulk temperature 80°C and flow velocity 0.1 m/s, the surface temperature is 85°C, the corrosion rate is 0.10 mm/year, and the decomposition rate is 0.5 g/m2·h. After 100 hours, the surface temperature rises to 90°C, the corrosion rate climbs to 0.15 mm/year, and the decomposition rate rises to 0.8 g/m2·h. The surface temperature reaches 95 °C at 200 h. The corrosion rate is 0.22 mm/year and the decomposition rate is 1.2 g/m 2 ·h. At 500 hours the surface temperature is 105°C, the corrosion rate is 0.35 mm/year and the breakdown rate is 2.0 g/m2h. At 1000 hours the surface temperature is 120°C, the corrosion rate is 0.60 mm/yr and the decomposition rate is 3.5 g/m2.h.

Chlorate Heaters Flow Velocity and Temperature Control Guide

The following table gives recommended minimum flow velocity and maximum allowable temperature rise for Grade 2 titanium heaters in 4% sodium chlorate at 80° C.

Desired Corrosion Rate (mm/year) Minimum Flow Velocity (m/s) Expected Local Temperature Rise (°C) Maximum Surface Temperature (°C) Recommended Action <0.10 >1.5 <2 <820.10–0.15 1.0–1.5 2–4 82–84 OK 0.15–0.25 0.5–1.0 4–8 84–88 Watch temperature 0.25–0.35 0.2–0.5 8–15 88–95 Speed up flow0.35 <0.2 >15 >95 Re-design system
Engineering Beyond Flow Rate Control

The rate of catalytic breakdown depends on the quality of titanium. The Grade 7 catalyst was as active as the Grade 2 catalyst. Activity is slightly lower for Grade 12. Wall thickness offers a corrosion allowance . A 2.0 mm wall with 0.20 mm per year corrosion will last 10 years . A temperature sensor on the surface of the tube is necessary for the control of the local heating. If the surface temperature is >90°C, reduce the chlorate concentration or increase the flow velocity. An inhibitor such as sodium dichromate (100 ppm) lowers the breakdown rate by 50–70 %.

Producing an informed specification:

For a titanium tube bundle heater preheating 4% sodium chlorate at 80°C, the flow velocity should be kept above 1.5 m/sec to prevent excessive local temperature rise and corrosion rate of 2°C and 0.10 mm per year, respectively. Fit a surface thermocouple to the heater to measure its temperature. If the temperature goes above 85C, turn up the circulation pump. For existing thermal runaway heaters add sodium dichromate (100 ppm) as breakdown inhibitor. Flow velocity control and temperature monitoring prevent thermal runaway and corrosion acceleration due to chlorate breakdown.