Monoethylene glycol (MEG) used as hydrate inhibitor in pipelines picks up corrosion products from carbon steel walls. This comes in addition to salts from produced fluids. Iron carbonate (siderite) scale can form on heat exchanger tubes when the MEG is heated and becomes supersaturated with respect to the solubility product of iron and carbonate. In this work a tube heat exchanger loop has been used to measure the scale growth rate of FeCO3 on hot surfaces simulating conditions typical for such heat exchangers. The scale growth rate was measured as function of skin temperature and FeCO3 supersaturation.

The temperature distribution in a tube heat exchanger is complex. The skin temperature is a function of the flow rate, temperature and heat capacities of the heating medium and the process fluids. In this work a tube heat exchanger model was used to calculate the skin temperature based on these parameters. The heat transfer to the test liquid depended on the MEG concentration in the liquid. The skin temperature in pure water was significantly lower than in MEG solutions at the same flow rate. This is mainly attributed to different viscosity. In the experiments, the flow rate on the MEG solution side was adjusted to achieve approximately the same Reynolds number for the various fluids to obtain comparable skin temperatures and flow patterns when the MEG concentration varied.

The FeCO3 supersaturation was adjusted to yield scaling only in the heated test section and avoid bulk nucleation and growth as well as scaling on other parts of the loop. MEG clearly decreases the scaling rate and the scale growth rate (Gscale) as a function of the supersaturation followed the equation:
Gscale=kg(Sskin1)1.5
where kg is a MEG-dependent scale growth rate constant and Sskin is the supersaturation at the scale surface. The supersaturation is a function of the skin temperature, but the intrinsic effect of the skin temperature on the scaling rate was so small that it could be justified to ignore it in the expression.

The paper presents new knowledge on scale growth on hot surfaces in general and growth of iron carbonate in particular.

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