Effect of Pretreatment Process on Scale Formation in the Re-Boiler Section of Monoethylene Glycol Regeneration Plant

Abstract

Monoethylene glycol (MEG) regeneration plants often use pretreatment vessels to precipitate divalent cations, such as Fe2+, Ca2+, and Mg2+, in order to avoid or reduce fouling in downstream reboilers and heat exchangers. This pretreatment process operates under alkaline conditions and moderate temperatures (~ 80 °C) to accelerate the formation of low-solubility divalent salts. The objective of the present research was to determine whether the pretreatment process could be minimized, without negatively impacts on the MEG regeneration process from to the formation of scale on the heater bundle in the presence of low concentrations of divalent cations in the rich MEG stream. Scale formation was analyzed under MEG regeneration process conditions using a dynamic scale loop (DSL) test and verification experiments were performed in a MEG regeneration and reclamation pilot plant, both with and without pretreatment conditions. The scaling tendencies of several rich MEG–brine mixtures were evaluated at different pH pretreatment levels and dissolved CO2 concentrations. An evaluation temperature of 180 °C was chosen to match the skin temperature of the reboiler heater bundle during the MEG regeneration process. The experiments of pH 7.24 showed high amounts of precipitation scale within the reboiler due to high remaining concentrations of mineral ions. In addition, small concentrations of calcium and magnesium ions led to the precipitation of calcite, dolomite, and magnesium hydroxide on the reboiler bundle and within associated filtered outputs even when a pretreatment vessel was present. These results were confirmed by the differential pressure build-up and Scanning Electron Microscopy analyses for each experimental condition. Another interesting finding is that pH increased within the reboiler due to CO2 gas boiling off at high operating temperatures, thus contributing to increased alkalinity levels, which in turn promoted scale formation. These results indicate that pretreatment should not be reduced, even with divalent ion concentrations as low as ~ 5 ppm TDS, due to the harsh conditions within the reboiler heater bundle.