Analyze Water Treatment Scale Inhibitor - PBTCA
1. What is a scale inhibitor?
The scale inhibitor is a kind of medicament which has a poorly soluble inorganic salt capable of dispersing in water, prevents or interferes with precipitation and scaling of a poorly soluble inorganic salt on a metal surface, and maintains a good heat transfer effect of a metal device.
Scale inhibitors not only control scale, but also control corrosion products, slime, and sludge to some extent. A large amount of scale-forming material can be controlled by adding a small amount of scale inhibitor.
2. Second, the development and types of scale inhibitors
Early use of scale inhibitors is processed natural polymer products such as starch, sodium gluconate, tannin, sulfonated lignin and the like.
Natural scale inhibitors have the advantages of low cost and no pollution. However, the scale inhibition rate is low, so the amount of the agent is large, and the aging effect of the natural polymer cannot meet the increasingly high requirements of production.
The action of the scale inhibitor is not conducive to the increase of the concentration of circulating water. Therefore, the use is gradually reduced, not used alone, but combined with synthetic corrosion inhibitors.
In the 1960s, the development of new scale inhibitors was a synthetic or polymeric product scale inhibitor, which has a higher scale inhibition rate than natural scale inhibitors and can meet the requirements of higher scale inhibition. Its use concentration is only a few milligrams per liter, generally, less than 10, commonly used scale inhibitors have the following categories.
Commonly used carboxylic acids are polyacrylic acid, polymethacrylic acid, and hydrolyzed polymaleic anhydride.
A variety of binary or ternary copolymers has been developed in recent years. Copolymers such as acrylic acid-acrylate, acrylic acid-hydrolyzed polymaleic anhydride, acrylic acid-methacrylate-hydroxy methacrylate, acrylic acid-methacrylic acid-AMPS.
Commonly used are HEDP acid, ATMP acid, and EDTMPA.
Such as hexaol phosphate, polyoxyethylene phosphate.
Such as 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA).
3. Mechanism of action of scale inhibitor
It is generally believed that its scale inhibition mechanism is as follows.
Inorganic scale (such as calcium carbonate) crystals are arranged in a certain lattice when growing. The crystal is dense and relatively strong.
When a polycarboxylic acid or an organophosphate (phosphonic acid or organophosphate) scale inhibitor is contained in water, the group of the scale inhibitor has a chelation ability to the metal ion.
It interferes with the crystal of inorganic scale, causing the crystal lattice to be distorted and become an irregular crystal. This is the lattice distortion effect.
Lattice distortion causes a hard scale to become amorphous and soft. The crystal of the scale is not easy to grow, and there are a large number of voids in the scale layer, and the adhesion force to each other is poor, and it is easily washed away in the water flow and can be discharged together with the sewage.
Agents such as polyphosphates, organophosphates or polycarboxylic acids can capture calcium and magnesium ions in water to form stable complexes.
This is actually equivalent to reducing the concentration of calcium and magnesium ions in the water, ie reducing the chance of Ca2+ and CO32-binding to form CaCO3.
That is to say, it is equivalent to increasing the allowable concentration of calcium and magnesium ions in the circulating water, that is, increasing the solubility of the calcium and magnesium salts. The effect of complex solubilization allows more calcium carbonate to stabilize in water.
Condensation and dispersion
Anionic scale inhibitors (such as polycarboxylic acids) are dissociated in water to obtain negative ions capable of adsorbing into microcrystals of a scale salt such as calcium carbonate.
First, the microcrystallites are formed into a double electron layer, and further adsorbed on the molecular chain of the negative ions, so that the microcrystallites are negatively charged. Since a plurality of microcrystal grains on the molecular chain bear the same electric charge and repel each other, they cannot form large crystal grains, and it is difficult for the scale-forming salt to deposit on the metal heat transfer surface to form a scale layer.
The anion of the anionic scale inhibitor has a cohesive effect on the microcrystallites and can be dispersed throughout the water system to give an average dispersion.
This coagulation and dispersion cause the scale-forming salt microcrystals to be stably suspended in the water, which actually reduces the chance of micro grain collision growth, a formation of crystal nuclei, and precipitation so that the water can accommodate more scale-forming salts.
Regeneration-release membrane hypothesis
The polyacrylic scale inhibitor can form a film co-precipitated with the inorganic crystal particles on the metal heat transfer surface.
When the film is increased to a certain thickness, it is broken on the heat transfer surface, and a certain size of the scale layer leaves the heat transfer surface. The growth of the scale layer is suppressed due to the continuous formation and rupture of such a film.
Electric double layer
The organic phosphonate scale inhibition mechanism is due to the enrichment of the scale inhibitor in the diffusion boundary layer near the growth nucleus, forming an electric double layer and inhibiting the condensation of scale ions or molecular clusters on the metal surface.