Precipitation of halite from supersaline brine from Tønder formation

This project deals various treatments for geothermal brine, which is a highly salty solution containing more that 50 TDS (Total Dissolved Solids). This project focuses on the brine from Tønder formation (T=73°C), which has an extraordinarily high salt content NaCl ([C] = 300-310 g/l). According to the data, the brine is either saturated or very close to saturated with sodium chloride, which can give huge precipitation problems on a geothermal plant. Calculations have shown that, if the solution is saturated, cooling from ≈70°C to 30-40°C will result in precipitation of 5 g NaCl per liter solution. With a flow of 200 m3/h, that gives precipitation of 100 kg salt per hour. The purpose behind this project is to determine if the precipitation will take place, and how it can be avoided. Additionally, membrane filtration will be examined as a potential tool for removal of salt from brine before the re-injection. The composition of the brine has been studied in detail, but it can not with certainty be determined whether the solution is saturated or undersaturated. No matter the thermodynamic considerations, there are also kinetic factors that must be taken into account. According to the results, there is no precipitation of NaCl as a result of cooling. Neither the heat exchanger nor the system were afflicted by scaling problems, formation of coating or clogging. The were no problems with the flow of brine in the plant whatsoever. The absence of precipitation was confirmed by both concentration measurements (titration and ICP), conductivity measurements, and filtration experiments. It is possible that the precipitation takes place in the system, yet it can not be traced analytically, partially because of the relatively small amounts of salt to precipitate, and in part because of the large natural variation for the concentration measurements. Precipitation of salt in the bulk rather than on the surface would not be surprising, taking into account the brine’s high concentrations. Homogeneous nucleation is a frequent phenomenon in concentrated solutions. In any case, NaCl seems to remain in the brine, regardless of the precipitation scenario. However, the precipitation of salt due to cooling was refuted by both ultrafiltration studies and XRD analyzes, and can be considered to be an unlikely phenomenon to occur. Ceramic membranes have been found to be ineffective for removal of salt. This applies both to the simple mechanism of retention and to mechanism of increased precipitation due to the presence of fouling (cake filtration mechanism).