WATER TREATMENT
Threshold inhibitors temporarily delay the onset of precipitation. In a once-through system, untreated or uninhibited water passing through a condenser may take < 5 seconds to begin precipitating CaCO 3 in a given set of operating conditions. Alternatively, effectively treated water containing a few ppb of phosphonate or polymeric inhibitor may prevent the onset of precipitation by 30 seconds or more.
In a once-through cooling application, the residence time of water passing through the condenser is typically < 10 seconds. As such, treating a system with ppb levels of an effective polymer and / or phosphonate that extends the induction time to > 10 seconds would mean the difference between scaling and not scaling the condenser. By comparison, 3-10 ppm of additive on an active basis is needed to inhibit formation of scale in a recirculating system with a holding time of ≥24 hours.
The extent and duration of threshold inhibition are related to multople factors or conditions, including: the driving forces for precipitation( pH, temperature, concentrations of scale-forming ions, etc.), the particular efficacy of the threshold inhibitor, other water impurities( dissolved gas and / or suspended solids), rate of water concentration or evaporation and frequency of additive dosage.
Sequestration, the complexation of a metal ion so that it loses its reactive properties, is another important function of treatment additives, particularly polymers and phosphonates. Unlike threshold inhibition, it does not require either stoichiometry or specific functionality. Some additives commonly used for mineral scale control can sequester ions like calcium, magnesium and barium, preventing them from forming insoluble complexes with compounds like carbonate and sulfate.
A chelate is a coordination compound in which a central metal ion, such as Ca 2
+, is attached by coordinate links to two or more nonmetal atoms in the same molecule, called ligands. Thus, a chelating agent is an additive that links to a metal ion at two or more points within the agent molecule.
In practice, polymers like polycarboxylates, sulfonated copolymers and phosphonates act as chelating agents with most multivalent ions, due to the multiple anionic binding sites within the molecules. In common usage, chelation further implies a more permanent or substantive relationship between the ion and the ligand and refers to stoichiometric relationships between them.
Stabilisation refers to two distinct mechanisms. In colloidal stabilisation, precipitation in a fluid occurs, but the polymer additive prevents the agglomeration of particles > 1 μg in size. These particles are thus stabilised via electrostatic interactions with the polymer and remain suspended throughout the water phase. Colloidal stabilisation can fail due to physical or chemical changes in the fluid that result in particulate agglomeration beyond one micron in size and bulk settling of the precipitate.
‘ Stabilisation’ is also used as a synonym for sequestration, where a coordination complex between a polymer additive and soluble ions, or surface interaction between polymer and forming crystal lattices, occurs, preventing precipitation. Typically, phosphonates lack stabilisation properties.
Particulate dispersion is a suspension of particulates in an
QCM Digital Controller aqueous solution. It involves a mixture of finely divided particles, called the internal phase( often of colloidal size), being distributed in a continuous medium, called the external phase. These can be inorganic( e. g. CaCO 3
), organic( e. g. biomass) or a mixture.
Whilst phosphonates do not have true dispersion functionalities, polymers can be quite effective. Polymer composition and molecular weight are key determinants in deriving functionality for effective particulate dispersion.
The final mechanism is crystal habit modification. A crystal habit means the normal size and shape of a precipitated substance in a given set of environmental conditions. The formation of crystals like CaCO 3 and their deposition onto surfaces follow a process of nucleation, lattice formation and propagation, bulk precipitation and surface deposition.
Modifying crystal habit involves introducing a‘ poison’ or contaminating additive that disrupts normal lattice formation. This, in turn, yields crystals tending either to re-dissolve or to precipitate in abnormal forms. This effect tends to reduce cohesion of the crystals to each other( dispersion) and adhesion of crystals to system surfaces( scaling).
In some ways, crystal modification is the basis for mineral scale control using phosphonates and polymers. It impacts threshold inhibition by disrupting the growth and propagation of the crystal lattice such that crystalloids tend to redissolve. This
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