11092 Effects of Reaction Kinetics of H2S, CO2, and O2 on the Formation of Black Powder in Sales Gas Pipelines

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Product Number: 51300-11092-SG

ISBN: 11092 2011 CP

Author: Robin Susilo, Boyd R. Davis and Abdelmounam M. Sherik

Publication Date: 2011

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Carbon dioxide (CO₂), hydrogen sulphide (H₂S) and oxygen (O₂) gases dissolve in water condensate on carbon steel pipelines that react with the walls to form iron carbonate, iron sulphides, and iron oxides as the corrosion products, respectively. It is unknown whether these gases react individually or if there is a competition between them leading to a kinetically favorable reaction. The reaction rate is governed by the concentration of gases dissolved in water which is in turn controlled by the gas composition, diffusivity, solubility, and the intrinsic kinetics. The reaction rate and products when fine iron powder (-325 mesh) suspended in water exposed to dry acid gas (CO₂ and H₂S) and exposed to humid acid gas are studied.

Our findings show that water immersion approach for corrosion study without mixing has a mass transfer resistance that slows down the reaction rate significantly. This may not be observed in actual gas pipelines as the pipeline is exposed to thin water film only and the turbulence created by the gas flow induces mixing. Iron sulphide and carbonate formation are found to depend on the CO₂/H₂S ratio and time. Initially, the formation of iron carbonate is more dominant than iron sulphide. Higher the CO₂/H₂S ratio leads to the higher iron carbonate formation than iron sulphide. Iron sulphide becomes more dominant in a longer time scale for all CO₂/H₂S ratios. Iron sulphide is oxidized to elemental sulphur when O₂ is present.

Keywords: black powder, internal corrosion, sales gas, moisture content