Abstract |
The use of cooling fluid is imperative for heat transfer applications and has multiple
functions, such as protection against overheating or freezing. However, due to the heat
stress conditions that occur inside such systems, these fluids can also contribute to
problematic issues in the cooling system, such as scaling/deposition on the heat transfer
surfaces and corrosion of the metallic components. Therefore, a variety of chemical
additives are added in the coolant or their concentrates aiming to protect the metal
surfaces from corrosion, prevent precipitate scaling and deposition, and maintain the
pH at an acceptable levels. Silicate- and phosphate-based additives are among the most
common inorganic corrosion inhibitors, frequently used in combination with other
organic or inorganic inhibitors. Unfortunately, if the processes are not carefully
controlled, both silicate and phosphate can participate in scaling events, depleting the
system from active corrosion inhibitor, and harming the heat transfer efficiency.
Silicon dioxide plays a vital role in both the industrial and biological world. Thus,
silicic acid polymerization and silica scale inhibition are intensively studied processes
in aqueous media. The silica chemistry and behavior in non-aqueous or mixed
aqueous/non-aqueous media was thoroughly investigated in this work, within the scope
of engine coolant technology. Unwanted silica precipitation of silicate containing
coolants, which are usually mixtures of water and glycols (most commonly
monoethylene glycol, MEG), may obstruct coolant flow in car engines, leading to
engine overheating and breakdown. It is therefore imperative to understand the
formation mechanisms of these silicate precipitates in the operating conditions of a car
engine. As only limited knowledge is available in the relevant literature on the fate of
silicates in non-aqueous or mixed aqueous/non-aqueous media, compared to the
aqueous systems, this presents an intriguing subject both from an academic and
industrial point of view. The aim of our approach is the systematic study of the
polycondensation chemistry of silicate and the variables (physical or chemical) that
exert either positive or negative influence on this process.
Next to silicate precipitation, uncontrolled phosphate deposition in the car engine
may also hamper the heat transfer from engine to coolant, as a thermally insulating
layer can be formed on hot surfaces. The precipitation and deposition of insoluble
mineral precipitates on critical surfaces like heat exchangers due to the presence of
dissolved ions is a common problem in the use of water for industrial processes such as
cooling, boiler, desalination, and geothermal systems. The presence of metal ions, such
as Al3+, Fe3+, Cu2+ and Ca2+ even at low concentrations can cause the precipitation of
phosphate. The inverse solubility features of these precipitates allow immediate and
irreversible precipitation under the heat stress conditions occurring inside a car engine.
The metal ion sequestration approach was used in this work to control the formation of
each metal phosphate scale separately. Finally, the performance of selected sequestrants
was evaluated in the presence of different blends of metal ions.
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