In a Titanium Tube Bundle Heater Reboiling a 15% Monoethanolamine Solution at 110°C, How Does the CO₂ Loading (0.3 vs. 0.5 mole/mole) Shift the Degradation Mechanism from Erosion to Pitting?

The carbon dioxide (CO₂) loading is defined as the moles of CO₂ per mole of MEA in a reboiler for 15% monoethanolamine (MEA) at 110°C. The solution is less corrosive at low loading (0.3) and degradation is mainly by erosion from suspended particles. At high loading (0.5) the solution is more aggressive and the process of degradation changes from erosion to pitting corrosion due to the production of carbamates and bicarbonates. This change is due to the effect of the greater CO2 loading that lowers the pH and raises the concentration of aggressive species that locally assault the passive film.

Mechanism of Mechanism Shift with CO2 Loading

For modest CO2 injection of 0.3 the solution pH is about 9.5 and the passive film on titanium is stable. Solid particles such as iron sulphide or degradation products produce mechanical erosion. With a large CO2 injection (0.5), the pH lowers to 8.0-8.5. Increase in the amounts of carbamate (RNHCOOH) and bicarbonate (HCO_3^-). These species are able to complex titanium ions, and to locally disrupt the passive layer to initiate pits. The aggressive chemistry in the pit drives the attack after a pit has been formed. The transition from erosion to pitting changes the failure mode from uniform thinning to localized perforation.

Quantitative degradation rates and mechanisms as a function of CO_2 loading

The following has been established by controlled testing in 15% MEA at 110°C with a flow velocity of 2 m/s over 1,000 hours. At a loading of 0.2 CO2, the overall degradation rate is 0.03–0.06 mm/year, with 70% erosion, no pitting, and a consistent wear pattern on the surface. At 0.3 loading the rate is 0.05 - 0.10 mm/year, erosion is 60%, pitting density is 0 - 5 pits/cm 2 and a mixed mechanism is active. At a loading of 0.4 the rate is 0.08–0.15 mm/year, pitting becomes prevalent at 60 % of deterioration, pitting density is 10–30 pits/cm 2 , and pit depths approach 50–100 µm. At the critical threshold loading of 0.5, the rate is 0.12–0.25 mm/yr, pitting accounts for 80%, pitting density is 30–80 pits/cm², and pit depths are 100–250 µm. At a loading of 0.6, the rate is 0.20–0.40 mm/year, pitting is entirely dominant, the pitting density is >80 pits/cm2, and pit depths are >250 mm.

Transition loading as a function of flow velocity and temperature

The transition loading at which pitting becomes dominant is a function of flow velocity and temperature. with 90°C, the change from erosion to pitting takes place with a CO₂ loading of 0.6. The transition occurs at 0.4–0.5 at 110°C. The transition is lowered to 0.3-0.4 at 130°C. The transition loading is 0.35–0.45 for a low flow velocity (0.5 m/s). Erosion at high flow velocity (3 m/s) is the main mechanism up to a loading of 0.55 since the mechanical removal of passive film precludes the onset of pitting.

MEA Reboiler CO2 Loading Control Guide

The following table gives recommendations of maximum CO2 loading to avoid pitting dominated degradation for Grade 2 titanium tube bundle reboilers at 110°C for desired service life.

Desired Service Life (years) Maximum CO2 Loading (mole/mole) Dominant Mechanism Expected Total Degradation Rate (mm/year)10 <0.3 Erosion <0.08 5-10 0.3-0.4Mixed 0.08-0.15 3-5 0.4-0.5 Pitting (marginal) 0.12-0.20 1-3 0.5-0.6 Pitting 0.15-0.30 <1 >0.6 Severe pitting >0.30
Engineering Past CO₂ Load Management

The transition loading is affected by the titanium grade. Grade 7 (palladium-stabilized) shifts the transition to pitting to CO2 loadings 0.1–0.2 above that of Grade 2. Grade 12 provides intermediate improvement. Pitting penetration is a function of wall thickness . At 2 . 0 mm wall and 0 . 2 mm / year pitting it will last 10 years . The MEA solution should be filtered to remove particles, which minimizes erosion but may increase the danger of pitting by allowing a stable passive film to form. Vanadium pentoxide and other corrosion inhibitors increase the transition loading. Lowering temperature to 100°C enables higher CO₂ loading without pitting.

Informed specification design

For a titanium tube bundle reboiler in 15% MEA at 110°C, the CO2 loading should be kept below 0.3 for Grade 2 or below 0.5 for Grade 7 to control pitting as a secondary degradation mechanism. "Monitor the CO2 loading weekly by titration. For Grade 2, if loading is above 0.4, decrease the lean MEA circulation rate or increase the stripping steam flow to decrease the loading. If your heaters are already pitting, you might want to convert to Grade 7 titanium or lower the temperature to 100°C. The engineer keeps the CO₂ loading below the transition threshold to prevent a change from predictable erosion to unpredictable pitting corrosion.