What is the primary mechanism by which scale inhibitors like polyphosphates prevent scale formation in MSF plants?

The primary mechanism by which scale inhibitors like polyphosphates prevent scale formation in Multi-Stage Flash (MSF) plants is through crystal distortion and threshold inhibition. Polyphosphates, when added to the recirculating brine, interfere with the nucleation and growth of scale-forming crystals, such as calcium carbonate (CaCO3) and magnesium hydroxide (Mg(OH)2). Nucleation is the initial formation of tiny crystal seeds from dissolved ions in the water. Polyphosphates adsorb onto these crystal nuclei, distorting their crystal lattice structure and preventing them from growing into larger, adherent scale deposits. This crystal distortion makes the scale less likely to adhere to heat transfer surfaces and remain suspended in the water, where they can be removed through blowdown. Threshold inhibition refers to the ability of polyphosphates to prevent scale formation even at concentrations far below the stoichiometric amount needed to react with all the scale-forming ions. They achieve this by disrupting the crystal growth process at very low concentrations. For example, a few parts per million (ppm) of polyphosphate can effectively inhibit the formation of calcium carbonate scale, even when the water is supersaturated with calcium and carbonate ions. Polyphosphates can also act as dispersants, preventing the aggregation of small crystals into larger particles. This helps to keep the crystals suspended in the water and reduces their tendency to deposit on heat transfer surfaces. While other scale control methods, such as acid dosing, aim to prevent supersaturation, polyphosphates allow the system to operate in a slightly supersaturated condition while preventing the formation of problematic scale deposits.