The Foam Problem in Water Treatment – Relevant Knowledge about Defoamers
The problem of foam in water treatment has troubled many people. There are various types of foam, such as foam in the initial debugging stage, foam caused by surfactants, impact foam, peroxide foam, and the foam generated when adding non-oxidizing bactericides in circulating water treatment, and so on. Therefore, the use of defoamers in water treatment is quite common. This article comprehensively introduces the principles, classifications, model selection, and dosage of defoamers!
The principle of defoaming
Defoamers, also known as antifoaming agents, have the following principles:
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The local reduction of the surface tension of the foam leads to the collapse of the foam.
The origin of this mechanism is that when higher alcohols or vegetable oils are sprinkled on the foam, when they dissolve into the foam liquid, the surface tension at that place will be significantly reduced. Since these substances generally have low solubility in water, the reduction of the surface tension is limited to the local area of the foam, while the surface tension around the foam hardly changes. The part with reduced surface tension is strongly pulled and extended in all directions, and finally breaks. -
Destroying the membrane elasticity leads to the bursting of the bubbles.
When the defoamer is added to the foam system, it will diffuse towards the gas-liquid interface, making it difficult for the surfactant with the function of stabilizing the foam to restore the elasticity of the membrane. -
Promoting the drainage of the liquid film.
The defoamer can promote the drainage of the liquid film, thus leading to the bursting of the bubbles. The drainage rate of the foam can reflect the stability of the foam. Adding a substance that accelerates the drainage of the foam can also play a role in defoaming. -
Adding hydrophobic solid particles can lead to the bursting of the bubbles.
On the surface of the bubbles, the hydrophobic solid particles will attract the hydrophobic ends of the surfactants, making the hydrophobic particles hydrophilic and entering the aqueous phase, thus playing a defoaming role. -
Solubilizing the foam-promoting surfactant can lead to the bursting of the bubbles.
Some low-molecular substances that can be fully mixed with the solution can solubilize the surfactant on the surface of the bubbles and reduce its effective concentration. Low-molecular substances with this effect, such as alcohols like octanol, ethanol, and propanol, can not only reduce the concentration of the surfactant in the surface layer, but also dissolve into the adsorption layer of the surfactant, reducing the tightness between surfactant molecules, thus weakening the stability of the foam. -
Electrolytes disintegrate the double electric layer of the surfactant.
For the foaming liquid that obtains stability through the interaction of the double electric layers of the surfactants in the foam, adding common electrolytes can disintegrate the double electric layer of the surfactant and play a defoaming role.
Classification of Defoamers
Commonly used defoamers can be classified into silicone (resin) types, surfactant types, paraffin hydrocarbon types, and mineral oil types according to different components.
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Silicone (Resin) Types
Silicone resin defoamers are also known as emulsion-type defoamers. The usage method is to emulsify and disperse the silicone resin in water with an emulsifier (surfactant) and then add it to the wastewater. Fine silica powder is another type of silicone defoamer with good defoaming effect. -
Surfactant Types
This type of defoamer is actually an emulsifier. That is, by using the dispersing effect of the surfactant, the substances that form the foam are stably emulsified and dispersed in water, thus avoiding the generation of foam. -
Paraffin Hydrocarbon Types
Paraffin hydrocarbon defoamers are defoamers made by emulsifying and dispersing paraffin wax or its derivatives with an emulsifier. Their uses are similar to those of the emulsion-type defoamers of the surfactant type. -
Mineral Oil Types
Mineral oil is the main defoaming component. To improve the effect, substances such as metal soaps, silicone oils, and silica are sometimes mixed and used together. In addition, in order to make the mineral oil easily diffuse to the surface of the foaming liquid, or to make the metal soaps evenly dispersed in the mineral oil, various surfactants are sometimes added.
Selection of Defoamers
The selection of defoamers should meet the following points:
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Insoluble or Sparingly Soluble in the Foaming Liquid
To break the foam, the defoamer should be concentrated and gathered on the foam film. In the case of a foam-breaking agent, it should be concentrated and gathered instantaneously, and in the case of an anti-foaming agent, it should always maintain this state. Therefore, the defoamer is in a supersaturated state in the foaming liquid. Only when it is insoluble or sparingly soluble can it easily reach the supersaturated state. Only when it is insoluble or sparingly soluble can it easily accumulate at the gas-liquid interface, easily concentrate on the foam film, and function at a low concentration. For defoamers used in water systems, the molecules of the active ingredients must be strongly hydrophobic and weakly hydrophilic, and the HLB value should be in the range of 1.5-3 for the best effect. -
Surface Tension Lower than that of the Foaming Liquid
Only when the intermolecular force of the defoamer molecules is small and the surface tension is lower than that of the foaming liquid can the defoamer particles immerse and spread on the foam film. It should be noted that the surface tension of the foaming liquid is not the surface tension of the solution, but the surface tension of the foam-promoting solution. -
Having a Certain Degree of Affinity with the Foaming Liquid
Since the defoaming process is actually a competition between the foam collapse rate and the foam generation rate, the defoamer must be able to disperse quickly in the foaming liquid so as to rapidly exert its effect within a relatively wide range of the foaming liquid. In order to make the defoamer diffuse more quickly, the active ingredients of the defoamer must have a certain degree of affinity with the foaming liquid. If the active ingredients of the defoamer are too closely related to the foaming liquid, they will dissolve; if they are too poorly related, they will be difficult to disperse. Only when the degree of affinity is appropriate can the defoamer be highly effective. -
Not Reacting Chemically with the Foaming Liquid
If the defoamer reacts with the foaming liquid, on the one hand, the defoamer will lose its function, and on the other hand, harmful substances may be generated, affecting the growth of microorganisms. -
Low Volatility and Long-lasting Effect
First, it is necessary to determine the system in which the defoamer needs to be used, whether it is an aqueous system or an oily system. For example, in the fermentation industry, oily defoamers such as polyether-modified silicone or polyether types should be used. In the water-based coating industry, water-based defoamers, such as silicone defoamers, should be used. Select the defoamer, compare the addition amounts, and refer to the prices to obtain the most suitable and economical defoamer product.
Dosage of Defoamers
There are many types of defoamers, and the required addition amounts of different types of defoamers are different. Below, we will introduce the addition amounts of six types of defoamers for you:
- Alcohol-based Defoamers: When using alcohol-based defoamers, the dosage is generally within 0.01-0.10%.
- Oil-based Defoamers: The addition amount of oil-based defoamers is between 0.05-2%, and the addition amount of fatty acid ester defoamers is between 0.002-0.2%.
- Amide-based Defoamers: Amide-based defoamers have a relatively good effect, and the addition amount is generally within 0.002-0.005%.
- Phosphoric Acid-based Defoamers: Phosphoric acid-based defoamers are most commonly used in fibers and lubricating oils, and the addition amount is between 0.025-0.25%.
- Amine-based Defoamers: Amine-based defoamers are mainly used in fiber processing, and the addition amount is 0.02-2%.
- Ether-based Defoamers: Ether-based defoamers are more commonly used in papermaking, printing and dyeing, and cleaning, and the addition amount is generally 0.025-0.25%.
