THE EFFECTS OF CHEMICAL ADDITIVES ON MAGNETITE DEPOSITION IN BOILING HEAT TRANSFER

This paper investigates the effects of three polymeric dispersants – two types of polyacrylic acid (PAA) and sodium polymethacrylate (PMA) – and a commercial filmforming amine (FFA) on the deposition onto heated nickel alloy of particulate magnetite from suspension in water at about 100oC 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 stainless steel recirculating loop with an electrically-heated heat-exchanger tube of Alloy-800. This latter is exposed to flowing water under the chemistry conditions of the bench experiments. A laser micrometer/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. Due to its agglomerating effect on magnetite, one can conclude little about the effect of FFA on magnetite deposition. INTRODUCTION Typical power-plant heat exchangers, along the feedtrain from the condenser, for example, raise the feed-water temperature in stages to improve the thermal efficiency. Many other water-cooled exchangers operate at relatively low temperatures for systems other than the main steam cycle. The fouling of these units by corrosion products can have serious implications for the operation of the plant, both in terms of the loss of efficiency and the potential for underdeposit corrosion. Adding a chemical to the boiler feed-water can reduce fouling. By adsorbing on walls and corrosion-product colloids, the molecules affect surface properties and thus modify fouling mechanisms. Three additives of interest are studied: a commercial film-forming amine (FFA) and the two polymers polymethacrylic acid (PMA sodium salt of molecular weight 9500) and polyacrylic acid (two versions – pure PAA of molecular weight 1800 and PAA sodium salt of molecular weight 8000). Ramminger et al. (2012) showed how FFA in the secondary circuit of a nuclear plant reduced the formation of corrosion products and their transport into the steam generator. They reported that FFA protects the metal by creating a hydrophobic film on its surface that acts as a barrier to charge and/or mass transport. Its effect is particularly beneficial during outages, when moisture and oxygen ingress can lead to rapid corrosion. The polymeric additives PMA and PAA have both reduced magnetite deposition rates under flow-boiling conditions by up to 75% and 80%, respectively, (Balakrishnan et al. 1999, Burgmayer et al. 1998), probably by stabilising magnetite suspensions in the bulk fluid at higher concentrations than in the absence of additive. Balakrishnan (1999) also found that between two types of PAA, the lower molecular weight was the more efficient at reducing deposition. For the present study, low and very low molecular weight PAA (respectively 8000 and 1800) and low molecular weight PMA (9500) are tested. These additives are studied in three steps: zeta potential analyses on magnetite and Alloy-800 particles and two types of fouling experiment at low pressure and low temperature. In eleven bench-top fouling experiments, ribbons of nichrome immersed in a suspension of magnetite are heated electrically. Deposition is analysed visually and photographically and with scanning electron microscopy (SEM). Seven complementary experiments are carried out in a stainless steel recirculating loop with a vertical glass column as test section. An electrically-heated heatexchanger tube of Alloy-800 is mounted in the top of the column and exposed to flowing water under the chemistry conditions of the bench experiments but under flow-boiling heat transfer. A laser micrometer/profilometer indicates the thickness of the deposited magnetite at the end of the run. The loop experiments are conducted with the polymeric dispersants only. All the experiments and analyses presented in this paper are carried out with magnetite synthesized by the solid-state method (Mohajery et al., 2012) and characterized with SEM. The magnetite particle size distribution ranges from 300 nm to 600 nm for the dry powder. Its purity was established using X-ray diffraction measurements to verify the species stoichiometry. Proceedings of International Conference on Heat Exchanger Fouling and Cleaning 2013 (Peer-reviewed) June 09 14, 2013, Budapest, Hungary Editors: M.R. Malayeri, H. Müller-Steinhagen and A.P. Watkinson Published online www.heatexchanger-fouling.com