A water sample from KO drum (downstream to HP absorber at sweating unit) is black in colour, rotten smelling, pH = 10, Fe = 4 mg/l... MDEA used for sweating, CO2 at feed gas about 3.5%, H2S nil. What is the reason for the black colour and rise in pH?Aug-2021
Sankar Nallasivamn, ADNOC Onshore, email@example.com
MDEA is alkaline substance, it is used to absorb H2S, so if stotiometric quantity of MDEA is high enough to create pH-10.
Any plant initial start-up in the refinery, the overhead water becomes black due to FeS absorption in the water and rotten egg smell is common.
What is the acceptable limit of iron Fe in your unit and adjust any chemical injection in the overhead system. Usually either ammonia or corrosion inhibitor will reduce iron content in the KO drum.
DOUGLAS DEWITT-DICK, Global Water Experts, firstname.lastname@example.org
The hydrogen sulphide is reacting with the soluble iron in the water phase to form iron (ferrous) sulphide (FeS). Iron sulphides may be iron deficient (non-stoichiometric) and all are black in colour and water insoluble.