Scale is a term used to describe inorganic mineral components of water that have formed as hard, adhering deposits over time.
Scale on heat exchanger surfaces is also referred to as fouling or residue.
These are highly general phrases, but they relate to deposits that form on heat exchanger surfaces as a result of corrosion products, liquid solidification, microbiological, particulate and organic chemical reactions as well as precipitation, which limit heat transfer and increase liquid flow resistance.
Scale inhibition has transformed over the past 50 years from an art to a science.
Scale inhibition has evolved from simple pH adjustments to the usage of designer polymers made from several monomers in an optimum dose.
What is a scale inhibitor?
Scale inhibitor is a chemical or mechanical treatment that is injected or installed in fluid flow systems to avoid the deposition and aggregation of slightly insoluble compound formation on the walls of the systems.
It is usually small and is intended to protect a single piece of equipment, such as a boiler.
However, it appears that many of these systems are available.
It consists of a technology that is specifically designed to decrease the rate of scale formation to promote water, gas and oil production industries.
It has the goal that improving the heat transfer and carrying capacity of the fluid distribution systems.
What are the different kinds of scale inhibitors?
Organic and inorganic scale inhibitors are the two broad categories.
Inorganic scale inhibitors phosphates include poly (metaphosphate) and phosphate salts.
Organic scale inhibitors that work well include poly (acrylic acid) (PAA), phosphine carboxylic acid, sulfonated polymers and phosphonates.
Scale inhibitors are categorized into three basic groups:
1) Thermodynamic inhibitors
Thermodynamic inhibitors are complex and chelating agents that are useful at different scales. Ethylene diamine tetra-acetic acid is a common chemical used to inhibit scale formation in the barium sulfate (EDTA).
Nitrilotriacetic acid changing the pH or partial pressure of carbon dioxide can affect calcium carbonate solubility (CO2).
The dissolution enthalpy limits the temperature coefficient of solubility.
As the temperature rises, the exothermic enthalpy of dissolution decreases, and vice versa.
2) Kinetic inhibitors
Kinetic hydration inhibitors have been developed to avoid these hydrate forms, allowing fluids to be pumped out before a catastrophic hydrate formation occurs.
Kinetic inhibitors hinder crystal development by inhibiting or delaying the formation of hydrate crystals.
These kinetic inhibitors are considered to impede hydrate crystal formation by being integrated into developing hydrate crystals and so interfering with future hydrate crystal growth.
The developing hydrate crystals complete a cage by joining with the incomplete hydrate-like cages around the kinetic hydrate inhibitor moieties bearing gas-like groups.
Water dissolves hydrophilic scale inhibitors. To avoid washing off the chemical before it can perform its function, the scale inhibitor should be adsorbed on the rock.
Adsorption on the rock, on the other hand, may produce changes in the surface tension and wettability of the system.
Another inhibitor mechanism is based on the adsorption mechanism, which argues that morphologic alterations can impede crystal formation in the presence of the inhibitor.
Depending on the inhibitor’s characteristics and the structure of the substrate, the inhibitor may be adsorbed across the crystalline net, generating complicated surfaces or nets that have trouble staying and developing inactive areas.
3) Adherence inhibitors
Another scale inhibition approach is based on adherence inhibitors, in which surface-active compounds simply prevent crystals from adhering to metal surfaces.
There are criteria for scaling inhibitor selection
• Stability of efficiency Compatibility.
• Inhibitors must not react with other substances or be influenced by them.
Detailed parameters evaluated while selecting scale inhibitor candidates for performance tests:
Scale type:
The optimal scale inhibitors chemical should be chosen based on the composition.
Scaling severity:
At high scaling rates, fewer items are effective.
Total cost:
Less costly items are sometimes the most cost-effective; other times, more expensive products are.
Temperature:
Higher temperatures and the necessity for longer life restrict the sorts of chemistry that are appropriate.
pH:
Most traditional scale inhibitors are less efficient in low-pH environments.
Chemical compatibility:
Scale inhibitors must be chemically compatible with other treatment chemicals, such as oxygen scavengers, corrosion inhibitors, and anti-corrosion agents.
How do scale inhibitors work?
Inhibitors work through several distinct mechanisms to lower size. Many claims to use techniques that are comparable to those of water conditioners.
Most scale inhibitors have electric or magnetic systems, while there are also several electrolytic methods available.
Advantages of scale inhibitors
– Provides corrosion inhibition in a variety of closed recirculating systems.
– Avoid electrolytic corrosion.
– Prevent cavitation and erosion.
– Shield metal surfaces
– Low cost, simple application, and usage
– Improved performance
– Assist in lowering cleaning and maintenance expenses
– Increase dependability
– Optimize operational efficiency
Limitations of scale inhibitors
– Because of temperature, pH, solution quality, concentration, phosphate type, and the presence of particular enzymes, inorganic corrosion and scale inhibitors are hydrolyzed and can precipitate as calcium phosphates.
– Organic corrosion and scale inhibitors are hydrolyzed with temperature, are ineffective at high calcium concentrations, and must be used in large quantities.
– Although certain polymer-based corrosion and scale inhibitors can operate at doses higher than 5000 ppm, bigger concentrations are required.
– EDTA is costly.
What causes scales?
The buildup of mineral scale is brought on by heat transfer or pressure fluctuations.
Examples include calcium phosphate and oxalate development in sugar refineries as well as calcium carbonate scaling from hard water.
Biofouling, which is the growth of bacteria and algae, and the collection of loose particles are other types of fouling (e.g., particulate fouling— corrosion byproducts and the accumulation of “coke-like” deposits).
What might go wrong?
Deposits act as insulating layers on surfaces used for heat transfer. To make up for this, either more energy is consumed or more powerful, costly heat exchangers are installed.
In a system that is clogged with 1/4 inch of calcium carbonate scale, it is estimated that 40% more energy is required to heat water.
Overheating caused by scaled boiler tubes causes mechanical failure, and the weight of scale deposits can cause cooling tower plates to fall.
When scale particles become free and then come into contact with other surfaces, erosion damage can follow.
Temporarily reducing the corrosivity of steel by forming a thin, homogeneous layer of scale or wax is possible, but over time stagnant conditions under the deposit lead to electrochemical processes that erode the steel surfaces.
The health and safety of the workforce may be in danger, in addition to the integrity of the plant and the products. In an emergency, clogged safety valves or emergency process sensors may not work.
Suppose bacterial growth in cooling water is not controlled. In that case, it may lead to potentially harmful conditions (such as the growth of Legionella pneumophila) or, in anaerobic situations, it may allow sulfate-reducing bacteria to produce poisonous hydrogen sulfide.
Reference
[1] Troy Adam, August 16, 2021, Corrosion and Scale Inhibitors, GRT, (online) available https://globalroadtechnology.com/corrosion-and-scale-inhibitors/
[2] CORRION PEDIA, January 11, 2019, scale inhibitors (online) available at https://www.corrosionpedia.com/definition/1965/scale-inhibitor
[3] Nick, July 16, 2021, what is scale inhibitor?, Haylcan,(online) available at https://halcyanwater.com/what-is-a-scale-inhibitor/
[4] CHEM READY, Corrosion Inhibitors and Scale Inhibitors, (online), available at https://www.getchemready.com/corrosion-scale-inhibitors/
[5] Industrial Fouling (online), available at: https://www.wwdmag.com/industrial-water-wastes-digest/article/10917827/industrial-fouling
