Corrosion due to wet CO2.

[I2P]

friends,
I am working in a natural gas processing plant. I would like to know how to determine the water dewpoint of the natural gas mixture which contains 0.3380 mole % CO2. We need to know whether we could be facing corrosion downstream of the fin fan cooler due to wet CO2.
The Pressure is 60 to 65 bar, and the temperature is 45 to 47 deg. C.We also need to limit the temperature on the higher side because the gas from the after cooler goes to a glycol dehydration column, where a very high gas temperature will lead to water removal problems. Thanks in advance

 

[palusa]

hirenjbhatt,

water dew point temp. depends from the actual gas composition in terms of both HC and Wtr contents.

In particular, based on actual water amount in the gas (lb water/MMSCF @ 60F, 14.7 psia) you can use the McKetta Diagram (e.g. GPSA Eng Data Book Vol 2, Fig. 20-3) to estimate if your gas @ 45'C, 65 bar is above or below the water dew point.

Reversing the approach, using the same diagram @ 115F, 950psia (=45C, 65 bar) and a MW of 17.7 (mainly C1) you find that you'll be at saturation if in yr gas water equals/exceeds approx 85 lb/MMSCF.

As far as wet CO2 corrosion concerns, assuming to be at saturation as above, the coorosion rate (CR) in mm/y could be evaluated by the De Waard correlation:

log10(CR)=5.8 -1710/T + 0.67*log10(pCO2)

where:
T is in K
pCO2 = yCO2*P = partial pressure CO2

Using yr CO2 mol% and a gas with mainly C1, this correlation gives you a CR of 0.91 mm/y (corrected for 65 bar total system pressure).

Consider that, typically, De Waard correlation gives CR's overestimated of approx 30-50%.

Hope it helps.

palusa