The Effect of Alkalinity, Magnesium and Ferrous on the Precipitation of Calcium Carbonate at Pre-Treatment Conditions of Mono Ethylene Glycol

Abstract

Mono ethylene glycol (MEG) is used to prevent formation of gas hydrate in the pipes transporting natural gas. The MEG is regenerated in a closed MEG loop, where water and salts are removed. The divalent ions are harder to remove due to the size of the crystals when they precipitate. It could therefore be necessary with a pre-treatment process, where the divalent ions are separated from the MEG by adding alkalinity.

In this thesis a pre-treatment unit is mimicked in a continuously reactor. Two streams are added to the reactor, one of the alkalinity and the other for divalent ions. The conditions for the experiments are 80 °C and 50 wt% MEG. The aim of the thesis is to see how the alkalinity, magnesium and ferrous is affecting the precipitation of calcium carbonate in the pre-treatment unit.

It was found that higher concentrations of alkalinity reduced the amount of calcium that is deposited as scale on the reactor wall. However, the crystals were smaller, which could give separation problems. The polymorphs formed were vaterite and aragonite. Magnesium increased the amount of scaling on the reactor walls. The magnesium was inhibiting the formation of vaterite and promoting the formation of magnesian calcite. It also changed the morphology of aragonite to spherical crystals. For high magnesium concentration amorphous calcium carbonate was formed. Increased concentrations of magnesium showed an increase for magnesium adsorbed on the surface or integrated into the calcium carbonate crystals.

Ferrous promotes the formation of vaterite and modifies the morphology to thin plate-like crystals. It is also promoting the formation calcite. Aragonite was the dominating polymorph. In the presence of both magnesium and ferrous, vaterite is not formed and the amount of magnesium and ferrous adsorbed on the surface or integrated into the calcium carbonate has increased.