Inhibitor Selection for Iron Scale Control in MEG Regeneration Process

Abstract

Abstract The formation of iron-containing scale due to high dissolved iron content and increased pH (=8.5) has been observed in a monoethylene glycol (MEG) regeneration system. The control of iron scale deposition under these harsh conditions causes recurring problems in the re-boilers, requires frequent filter change-outs and remains a challenge to the field. To investigate this problem, a modified corrosion testing apparatus was employed to replicate the iron scale formation and to evaluate the performance of selected chemicals under realistic field conditions (specifically: low pH caused by carbon dioxide dissolution; higher pH due to carbon dioxide boiling off from the re-boiler and alkali addition for corrosion control). Both pH and iron levels (including dissolved iron, ferrous iron Fe2+ and total iron) were closely monitored; additionally digital photos were taken at various stages. There are several challenges associated with the inhibition of iron scale in systems with high iron level (=150 ppm), high MEG content (=60%) and high pH (=8.5). These factors contribute to incompatibility and poor inhibition by conventional inhibitor species. A number of scale-inhibitor products, scale dissolvers and their blends were tested under aerobic or anaerobic conditions. One product was identified to have greatly reduced the rate of scale formation and at higher dosage, indefinitely delayed scale formation. This product also showed the capability to re-dissolve deposits already present in the system. Evaluation of iron scale inhibitor is a challenge due to the debatable iron-poisoning effect and the lack of standard laboratory methodology. In this paper, the propensity of various iron deposits (e.g., iron carbonate, and insoluble iron/inhibitor complex) is discussed. Moreover, the formation of soluble iron/inhibitor complex is quantified; this could have important implications for the remediation of iron-containing scale. Introduction Iron scales can result from the accumulation of corrosion products (iron oxides, hydroxides) or can be derived from the interaction of soluble iron with its environment. It is well known that the inhibition of iron scales is a challenge as the inhibition efficiency of most common scale inhibitors are greatly affected by the presence of dissolved iron (Coleman et al., 1999; Dyer et al., 2000 and 2006; Graham et al., 2002, 2003a and 2003b; Johnson et al., 2005). However, there are several debates in this area of research as listed below: