Effect of monoethylene glycol on sweet top of the line corrosion

摘要

In wet gas pipelines, Monoethylene glycol (MEG) is a widely used hydrate inhibitor which has been shown to decrease the corrosion rate of carbon steel in CO_2 environments. In a top of the line corrosion (TLC) situation, MEG is also known to affect both water condensation and TLC rates. However, the extent of its effect on corrosion depends mainly on the concentration of MEG present in the condensed water. Until now, rather scarce and conflicting information exist on this topic. This work presents a mechanistic water/MEG co-condensation model in the presence of a noncondensing gas (CO_2). The model predictions of condensation rate and MEG concentration in the condensing phase are compared with loop test results, showing good agreement. The results show that an increase of the MEG content at the bottom of the line decreases the water condensation rate and increases the MEG content of the condensing phase at the top of the line. However, this effect is not significant unless the MEG content in the bulk liquid phase is higher than 70-80 wt%. Long term corrosion experiments are also presented showing that the injection of 50 wt% and 70 wt% MEG at the bottom have a minimal effect on both general and localized corrosion rates. On the other hand, the presence of 90 wt% MEG at the bottom of the line decreased the top of the line corrosion rate significantly due to a sharp decrease in condensation rate and a significant increase in MEG content in the condensing phase.