The effect of chemical additives on the deposition of magnetite onto alloy-800 under nucleate boiling heat transfer

Abstract

This thesis investigates the effects of three polymeric dispersants – two types of polyacrylic acid (PAA) and sodium polymethacrylate (PMA) – and a commercial film-forming amine (FFA) on the deposition onto heated nickel alloy of particulate magnetite from suspension in water during sub-cooled boiling and bulk boiling. In bench-top experiments, ribbons of Nichrome immersed in a magnetite suspension are heated electrically with and without additives. Results show that deposits are less numerous in the presence of polymeric additive. The smaller molecular weight PAA is the most effective at preventing magnetite deposition. Complementary experiments are carried out in a recirculating loop with an electrically-heated heat-exchanger tube of Alloy-800. The tube is exposed to flowing water under chemistry conditions comparable to those used during bench-top experiments. A laser profilometer is used to measure the thickness of the deposited magnetite at the end of each run. The use of polymeric dispersant is efficient at mitigating magnetite deposition. Again, the smaller molecular weight PAA is the most effective. A numerical model is developed to assist in the interpretation of the results from the laser profilometer. Due to its agglomerating effect on magnetite, one can conclude little about the effect of FFA on magnetite deposition.