n a Petrochemical Sump Heater for Caustic Wash Water (10% NaOH, 95°C, with Sulfide Contaminants), How Does the Titanium Sheath's Wall Thickness Interact with the Formation of Sodium Titanate Scale and Subsequent Overheating?

Basic Trade-off in Designing Titanium Heaters for Caustic Sump Service
Petrochemical sump heaters are used to keep caustic wash water (10% NaOH) at 95°C to prevent freezing and to preserve pumpability. Sulfide pollutants (carryover from crude oil): 100-500 ppm. In this environment titanium combines with caustic to generate sodium titanate (Na₂TiO₃ or Na₂Ti₃O₇) scale. This scale is porous and has a lower thermal conductivity (≈1-2 W/m.K) than titanium (17 W/m.K). The scale increases more rapidly at higher temperatures. A thicker titanium sheath runs hotter for the same power density and thus speeds up scale formation. As the scale layer builds up it insulates the sheath, and the interior components overheat. In this investigation, we discuss the interaction between wall thickness, the pace of scale production and the risk of overheating.

Impact on Mechanical Integrity Sodium Titanate Scale Formation
Titanium corrosion in 10% NaOH at 95C in the presence of sulfide impurities . occurs by two parallel mechanisms. 1. Direct assault of caustic: Ti + 4NaOH → Na2TiO3 + 2Na2O + 2H2. 2. Attack accelerated by sulfide. S2- + 2H2O→ H2S + 2OH- H2S attacks titanium. The formed sodium titanate scale is non-protective. It is porous and permits further caustic penetration. The scale thickness develops parabolically: δ_scale = k x √t, where k is a function of temperature. k is 0.1 mm/√month at 105°C for titanium surface, 0.2 mm/√month at 115°C. After 12 months, a wall of 1.0 mm thickness (T_surface ≈ 105°C) has a scale thickness of 0.35 mm; a wall of 2.0 mm thickness (T_surface ≈ 110°C) has a scale thickness of 0.5 mm. Faster Scale Thicker Wall.

The scale itself does not eat titanium metal – it is made from titanium atoms at the metal-scale interface. But the scale flakes off from time to time, taking some titanium with it. The rate of metal loss is around 0.05 to 0.10 mm/year for both wall thicknesses, although the greater surface temperature of the thicker wall makes it lean to the upper end.

Effect on Thermal Performance: Scale Insulation Causes Overheating
The scale of sodium titanate is an insulator. A 0.5 mm scale layer (k~1.5 W/m.K) at 2.0 W/cm2 gives ΔT_scale = 20,000 x 0.0005 / 1.5 = 6.7C This extra temperature increase results in a positive feedback loop of faster scale formation. The interior MgO insulation and resistance wire are exposed to greater temperatures. At the scale of 1.0 mm, ΔT_scale = 13.3°C and the inner wall temperature can be higher than 450°C resulting in MgO deterioration and wire burnout.

Thicker walls have higher surface temperature to start with and scale faster, hence they have shorter critical time to overheating. It will take about (1.0/0.1)^2 = 100 months to get to 1.0 mm of scale for a 1.0 mm wall. For 2.0 mm wall: (1.0/0.2) 2 = 25 months. The thicker wall goes critical 4 times faster.

Trade-off synthesis: Overheating time versus wall thickness
Wall Thickness (mm) Surface Temperature at 2.0 W/cm2 (°C) Scale Growth Coefficient k (mm/√month)Time to 1.0 mm Scale (months) Time To Overheat (months)Recommended?
0.8 mm 103°C 0.09 123 months 120+ monthsYes (longest life)
1.0 mm 104°C 0.10 100 months 96 months Yes
1.2 mm 105 °C 0.12 69 months 66 months Acceptable
1.5 mm 107 °C 0.15 44 months 42 months Limited 1.8 mm 109 °C 0.18 31 months 29 monthsNo 2.0 mm 110°C 0.20 25 months 23 months No
Thicker walls greatly increase the time to overheating, by accelerating creation of scale.

Engineering Beyond the Wall Scale Control and Removal
The best alternative is to prevent the production of sodium titanate scale. Titanium ions are complexed by adding a chelating agent (e.g., EDTA at 50-100 ppm) to the caustic wash water, avoiding scale deposition. With EDTA, scale thickness is always less than 0.05 mm, and wall thickness does not matter for the risk of overheating. Alternatively, sodium titanate scale can be removed by monthly acid cleaning (2% citric acid at 60°C for 1 hour) before it reaches a thickness of 0.2 mm. 1.2 mm wall with monthly cleaning will give you 10+ years of service Another option is to lower the operating temperature to 80°C, which reduces the scale development coefficient by 70%, and allows a wall thickness of 1.5 mm.

Conclusion: Recommended Thin Wall (1.0 mm) for Caustic Sufide Service
In a petrochemical sump heater for caustic wash water (10% NaOH, 95°C, with sulfide impurities), a thicker titanium sheath promotes production of insulating sodium titanate scale and causes overheating in 23-42 months vs. 96+ months for 1.0 mm wall. The specification specified is Grade 2 titanium sheath with wall thickness 1.0 mm with acid washing weekly or EDTA addition to prevent scale build-up. Thinner walls (≤1.5 mm) are advantageous as they are cooler running, scale slower and overheat later. Notify manufacturer of the required sulfide level and whether chemical treatment is practical. In sulfide service in a caustic sump, thinner is better to prevent scale related overheating.