Antifoam Testing to Diminish Gas Entrainment in an Oil-Based Heating Medium

Abstract

Abstract An oil-based heating medium (HM) system was found to have a high level of nitrogen gas entrained in the fluids due to the surge drum configuration located in-line with the process flow. The HM surge drum acts as an expansion vessel to account for volume changes when heating from ambient temperature to the operating temperature (260°C). The surge drum is pressurized by supplying nitrogen to reduce the vapor pressure and to maintain inert operating conditions, and is the coldest part of the circuit running between 140-160°C. The return flow from process and utility users was found to "waterfall" into the surge drum. Sampling of the heating medium at the suction side of the discharge pumps drawing heat transfer fluid (HTF) from the surge tank, showed that almost 20 vol% of the heating medium, at standard conditions was nitrogen gas. Modelling showed that the extent of nitrogen gas entrainment would be dependent on the fluid level in the surge tank and would likely originate from bubbles in the size range of 600-1200 microns exiting the Expansion Tank. Gas entrainment in the HTF at elevated operational temperatures is a significant concern that can lead to inefficient heating of the HTF and resulting in a loss of heating control to downstream users. A chemical treatment solution, using an PDMS antifoam agent, was explored to mitigate gas entrainment in the HTF. Foam reduction was observed when the HTF was treated with an antifoam in Foam Column tests. However, the PDMS antifoam agent showed no efficacy in reducing gas entrainment in a modified Ross-Miles Test. The contrast in results was attributed to the fact that the antifoam agent was able to rupture films when applied to actual HTF foam films, however when applied to entrained bubbles dispersed in the HTF, there are no films on which the antifoam can act on. It was concluded that mechanical options are far more likely to mitigate gas entrainment.