Scale Management in Monoethylene Glycol MEG Systems - A Review

The paper reviews operational issues that arise when MEG is used for hydrate inhibition, especially when it is regenerated and recirculated. Thermodynamic equilibrium software can assess the scaling risk to some extent. Utilisation of data on nucleation and growth of scale formers like calcium carbonate can enhance the accuracy of the predictions. The presence of MEG and the conditions encountered in MEG systems favour aragonite crystallisation when the MEG solutions become supersaturated in CaCO3. MEG retards the growth rate of all three CaCO3 polymorphs, but the reduction is smaller for aragonite than for calcite and vaterite. The growth rate of siderite is also slowed down by MEG. However, MEG does not inhibit the nucleation and growth of carbonates. Alkalinity in recycled MEG will enhance the scaling risk downstream of the MEG injection point when there is calcium in the produced water. Scale can be mitigated by scale inhibitors, but the selection process must ensure that the chemicals are tested at relevant conditions; i.e. with the expected MEG concentration, alkalinity and pH. Many MEG recovery units have a pre-treatment system for controlled removal of carbonates and to some extent hydroxides. This reduces the amount of scale that may form in the regeneration system. In the pre-treatment, alkalinity dosed as hydroxide and/or carbonate forces precipitation of calcium, strontium and iron carbonates and magnesium hydroxide. The supersaturation is generally so high that scale inhibitors are not able to prevent precipitation of the solids.