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The principle, classification, selection, and dosage of defoamers

The foam problem in water treatment has puzzled many people. At the initial stage of commissioning, foam, surfactant foam, impact foam, peroxide foam, foam generated by adding non oxidizing bactericide in circulating water treatment, etc., so the use of defoamer in water treatment is relatively common. This article comprehensively introduces the principle, classification, selection and dosage of defoamer!

★ Elimination of foam
1. Physical methods

From the physical point of view, the methods to eliminate foam mainly include placing baffle or filter screen, mechanical agitation, static electricity, freezing, heating, steam, ray irradiation, high-speed centrifugation, pressure reduction, high-frequency vibration, instantaneous discharge and ultrasonic (acoustic liquid control). These methods all promote the gas transmission rate at both ends of the liquid film and the liquid discharge of the bubble film to varying degrees, making the stability factor of foam less than the attenuation factor, so that the number of foam gradually decreases. However, the common disadvantage of these methods is that they are highly constrained by environmental factors and have a low defoaming rate. The advantages are environmental protection and high reuse rate.

2. Chemical methods

The chemical methods to eliminate foam mainly include chemical reaction method and adding defoamer.

The chemical reaction method refers to the chemical reaction between the foaming agent and the foaming agent by adding some reagents to generate water insoluble substances, thus reducing the concentration of surfactant in the liquid film and promoting the rupture of foam. However, this method has some shortcomings, such as the uncertainty of foaming agent composition and the harm of insoluble substances to system equipment. The most widely used defoaming method in various industries nowadays is the method of adding defoamers. The biggest advantage of this method is its high defoaming efficiency and ease of use. However, finding a suitable and efficient defoamer is the key.

★The principle of defoamer

Defoamers, also known as defoamers, have the following principles:

1. The mechanism of foam local surface tension reduction leading to foam bursting is that higher alcohols or vegetable oils are sprinkled on the foam, and when dissolved into foam liquid, the surface tension will be significantly reduced. Because these substances generally have low solubility in water, the reduction of surface tension is limited to the local part of foam, while the surface tension around foam has almost no change. The part with reduced surface tension is strongly pulled and extended in all directions, and finally breaks.

2. The destruction of the membrane elasticity leads to the bubble breaking defoamer added to the foam system, which will diffuse to the gas-liquid interface, making it difficult for the surfactant with foam stabilizing effect to recover the membrane elasticity.

3. Defoamers that promote liquid film drainage can promote liquid film drainage, thus causing bubbles to burst. The foam drainage rate can reflect the stability of foam. Adding a substance that accelerates foam drainage can also play a role in defoaming.

4. Adding hydrophobic solid particles can cause bubbles to burst on the surface of the bubbles. Hydrophobic solid particles attract the hydrophobic end of the surfactant, making the hydrophobic particles hydrophilic and entering the water phase, thereby playing a role in defoaming.

5. Solubilizing and foaming surfactants can cause bubbles to burst. Some low molecular weight substances that can be fully mixed with the solution can solubilize the surfactant and reduce its effective concentration. Low molecular substances with this effect, such as octanol, ethanol, propanol and other alcohols, can not only reduce the surfactant concentration in the surface layer, but also dissolve into the surfactant adsorption layer, reducing the compactness of surfactant molecules, thus weakening the stability of foam.

6.Electrolyte breakdown surfactant double electric layer plays a defoaming role in the interaction of surfactant double electric layer with foam to produce stable foaming liquid. Adding ordinary electrolyte can collapse the surfactant double electric layer.

★ Classification of defoamers

The commonly used defoamers can be divided into silicone (resin), surfactant, alkane, and mineral oil according to their composition.

1. Silicone (resin) defoamers, also known as emulsion defoamers, are used by emulsifying and dispersing silicone resin with emulsifiers (surfactants) in water before adding it to wastewater. Silicon dioxide fine powder is another type of silicon-based defoamer with better defoaming effect.

2. Surfactants such defoamers are actually emulsifiers, that is, they use the dispersion of surfactants to keep foam forming substances in a stable emulsified state in water, so as to avoid the formation of foam.

3. Alkane based defoamers are defoamers made by emulsifying and dispersing paraffin wax or its derivatives using emulsifiers. Their use is similar to that of surfactant based emulsifying defoamers.

4.Mineral oil is the main defoaming component. To improve the effect, sometimes metal soap, silicone oil, silica and other substances are mixed together for use. In addition, various surfactants can sometimes be added to facilitate the diffusion of mineral oil onto the surface of the foaming solution or to evenly disperse metal soaps and other substances in the mineral oil.
★ Advantages and disadvantages of different types of defoamers

The research and application of organic defoamers such as mineral oils, amides, lower alcohols, fatty acids and fatty acid esters, phosphate esters, etc. are relatively early and belong to the first generation of defoamers. They have the advantages of easy availability of raw materials, high environmental performance, and low production costs; The disadvantages are low defoaming efficiency, strong specificity, and harsh usage conditions.

Polyether defoamers are second-generation defoamers, mainly including straight chain polyethers, polyethers starting from alcohols or ammonia, and polyether derivatives with end group esterification. The biggest advantage of polyether defoamers is their strong anti foaming ability. In addition, some polyether defoamers also have excellent properties such as high temperature resistance, strong acid and alkali resistance; The disadvantages are limited by temperature conditions, narrow application areas, poor defoaming ability, and low bubble breaking rate.

Organic silicone defoamers (third-generation defoamers) have strong defoaming performance, rapid defoaming ability, low volatility, no toxicity to the environment, no physiological inertia, and a wide range of applications. Therefore, they have broad application prospects and huge market potential, but their defoaming performance is poor.

Polyether modified polysiloxane defoamer combines the advantages of both polyether defoamers and organosilicon defoamers, and is the development direction of defoamers. Sometimes it can be reused based on its reverse solubility, but currently there are few types of such defoamers and they are still in the research and development stage, resulting in high production costs.

★ Selection of defoamers

The selection of defoamers should meet the following criteria:

1. If it is insoluble or insoluble in the foaming solution, it will break the foam. The defoamer should be concentrated on the foam film. For defoamers, they should be concentrated and concentrated in an instant, while for foam suppressants, they should be kept in this state regularly. So defoamers are in a supersaturated state in foaming liquids, and only insoluble or poorly soluble ones are prone to reaching supersaturation. Insoluble or difficult to dissolve, it is easy to aggregate at the gas-liquid interface, easy to concentrate on the bubble membrane, and can function at lower concentrations. The defoamer used in water systems, the active ingredient molecules, must be strongly hydrophobic and weakly hydrophilic, with an HLB value in the range of 1.5-3 for the best effect.

2. The surface tension is lower than that of the foaming liquid, and only when the intermolecular forces of the defoamer are small and the surface tension is lower than that of the foaming liquid, can the defoamer particles penetrate and expand on the foam film. It is worth noting that the surface tension of the foaming solution is not the surface tension of the solution, but the surface tension of the foaming solution.

3. There is a certain degree of affinity with the foaming liquid. As the defoaming process is actually a competition between the foam collapse speed and the foam generation speed, the defoamer must be able to quickly disperse in the foaming liquid so as to quickly play a role in a wider range of foaming liquid. To make the defoamer diffuse quickly, the active ingredient of the defoamer must have a certain degree of affinity with the foaming solution. The active ingredients of defoamers are too close to foaming liquids and will dissolve; Too sparse and difficult to disperse. Only when the closeness is appropriate can the effectiveness be good.

4. Defoamers do not undergo chemical reactions with foaming liquids. When defoamers react with foaming liquids, they lose their effectiveness and may produce harmful substances that affect microbial growth.

5.Low volatility and long duration of action. Firstly, it is necessary to determine whether the system that requires the use of defoamers is water-based or oil-based. In the fermentation industry, oil-based defoamers such as polyether modified silicone or polyether based ones should be used. The water-based coating industry requires water-based defoamers and organic silicon defoamers. Select the defoamer, compare the amount added, and based on the reference price, determine the most suitable and economical defoamer product.

★Factors affecting the effectiveness of defoamer use

1. The dispersibility and surface properties of defoamers in solution significantly affect other defoaming properties. Defoamers should have an appropriate degree of dispersion, and particles that are too large or too small in size can affect their defoaming activity.

2. Compatibility of defoamer in foam system When the surfactant is completely dissolved in aqueous solution, it is usually directionally arranged on the gas-liquid interface of foam to stabilize foam. When the surfactant is in insoluble or supersaturated state, the particles disperse in the solution and accumulate on the foam, and the foam acts as defoamer.

3. The ambient temperature of the foaming system and the temperature of the foaming liquid can also affect the performance of the defoamer. When the temperature of the foaming liquid itself is relatively high, it is recommended to use special high temperature resistant defoamer, because if ordinary defoamer is used, the defoaming effect will certainly be greatly reduced, and the defoamer will directly demulsify the lotion.

4. The packaging, storage, and transportation of defoamers are suitable for storage at 5-35 ℃, and the shelf life is generally 6 months. Do not place it near a heat source or expose it to sunlight. According to commonly used chemical storage methods, ensure sealing after use to avoid deterioration.

6.The addition ratio of defoamers to the original solution and diluted solution has some deviation to a certain extent, and the ratio is not equal. Due to the low concentration of surfactant, the diluted defoamer lotion is extremely unstable and will not delaminate soon. The defoaming performance is relatively poor, which is not suitable for long-term storage. It is recommended to use immediately after dilution. The proportion of defoamer added needs to be verified through on-site testing to evaluate its effectiveness, and should not be excessively added.

★The dosage of defoamer

There are many types of defoamers, and the required dosage for different types of defoamers varies. Below, we will introduce the dosage of six types of defoamers:

1. Alcohol defoamer: When using alcohol defoamers, the dosage is generally within 0.01-0.10%.

2. Oil based defoamers: The amount of oil based defoamers added is between 0.05-2%, and the amount of fatty acid ester defoamers added is between 0.002-0.2%.

3. Amide defoamers: Amide defoamers have a better effect, and the addition amount is generally within 0.002-0.005%.

4. Phosphoric acid defoamer: Phosphoric acid defoamers are most commonly used in fibers and lubricating oils, with an added amount between 0.025-0.25%.

5. Amine defoamer: Amine defoamers are mainly used in fiber processing, with an added amount of 0.02-2%.

7.Ether based defoamers: Ether based defoamers are commonly used in paper printing, dyeing, and cleaning, with a typical dosage of 0.025-0.25%.