tel
+86-21-6420 0566
The foam problem in water treatment has troubled many people, such as the initial debugging foam, surfactant foam, impact foam, peroxide foam, foam generated by adding non-oxidizing bactericides in circulating water treatment, etc. Therefore, the use of defoamers in water treatment is relatively common. This article comprehensively introduces the principle, classification and selection of defoamers!
1. Physical methods
From a physical point of view, the methods for eliminating foam mainly include placing baffles or filters, mechanical stirring, static electricity, freezing, heating, steam, radiation, high-speed centrifugation, pressurization and decompression, high-frequency vibration, instantaneous discharge and ultrasonic waves (acoustic liquid control), etc. These methods have promoted the permeation rate of gas at both ends of the liquid film and the drainage of the bubble film to varying degrees, making the stability factor of the foam less than the attenuation factor, so that the amount of foam gradually decreases. However, the common disadvantages of these methods are that their use is strongly restricted by environmental factors and the defoaming rate is not high. The advantages are environmental protection and high reuse rate.
2. Chemical methods
The methods for eliminating foam from a chemical point of view mainly include chemical reaction method and the method of adding defoamers. The chemical reaction method refers to adding some reagents to react chemically with the foaming agent to generate water-insoluble substances, thereby reducing the concentration of surfactants in the liquid film and promoting the collapse of the foam. However, this method has the disadvantages of uncertain composition of the foaming agent and the generation of insoluble substances that harm the system equipment. Nowadays, the most widely used defoaming method in all walks of life is the method of adding defoaming agents. The biggest advantage of this method is high foam breaking efficiency and convenience of use, but finding a suitable and efficient defoaming agent is the key.
Defoaming agent, also known as defoaming agent, has the following principles:
1. The local surface tension of the foam is reduced, causing the foam to burst
The origin of this mechanism is to sprinkle higher alcohols or vegetable oils on the foam. When they are dissolved in the foam liquid, the surface tension at that place will be significantly reduced. Because these substances are generally less soluble in water, the reduction in surface tension is limited to the local part of the foam, while the surface tension around the foam has almost no change. The part with reduced surface tension is strongly pulled and extended to the surroundings, and finally breaks.
2. Destroy membrane elasticity and cause bubbles to burst
When defoamers are added to the foam system, they will diffuse to the gas-liquid interface, making it difficult for surfactants with foam stabilization to restore membrane elasticity.
3. Promote liquid membrane drainage
Defoamers can promote liquid membrane drainage, thereby causing bubbles to burst. The rate of foam drainage can reflect the stability of the foam. Adding a substance that accelerates foam drainage can also play a defoaming role.
4. Adding hydrophobic solid particles can cause bubbles to burst
Hydrophobic solid particles on the bubble surface will attract the hydrophobic end of the surfactant, making the hydrophobic particles hydrophilic and entering the water phase, thereby playing a defoaming role.
5. Solubilizing and foaming surfactants can cause bubbles to burst
Some low-molecular substances that can be fully mixed with the solution can solubilize the bubble surfactant and reduce its effective concentration. Low-molecular substances with this effect, such as alcohols such as octanol, ethanol, and propanol, can not only reduce the surfactant concentration on the surface layer, but also dissolve into the surfactant adsorption layer, reduce the tightness between surfactant molecules, and thus weaken the stability of the foam.
6. Electrolyte disintegrates the double electric layer of surfactants
For foaming liquids that generate stable foaming liquids by the interaction of the double electric layers of surfactants with foam, adding ordinary electrolytes can disintegrate the double electric layer of surfactants to defoam.
Common defoamers can be divided into silicone (resin), surfactant, paraffin and mineral oil according to their ingredients.
1. Silicon (resin)
Silicone resin defoamers are also called emulsion defoamers. The method of use is to emulsify and disperse silicone resin in water with an emulsifier (surfactant) and then add it to the wastewater. Silicon dioxide fine powder is another silicone defoamer with better defoaming effect.
2. Surfactant
This type of defoamer is actually an emulsifier, that is, it uses the dispersing effect of surfactants to keep the foam-forming substances in a stable emulsified state in water, thereby avoiding the formation of foam.
3. Alkanes
Alkanes defoamers are defoamers made by emulsifying and dispersing alkane wax or its derivatives with emulsifiers. Their uses are similar to those of surfactant-type emulsified defoamers.
4. Mineral oils
Mineral oil is the main defoaming component. In order to improve the effect, metal soaps, silicone oils, silicon dioxide and other substances are sometimes mixed and used together. In addition, in order to make it easier for mineral oil to diffuse to the surface of the foaming liquid, or to make metal soaps and the like evenly dispersed in the mineral oil, various surfactants can sometimes be added.
Mineral oils, amides, lower alcohols, fatty acids and fatty acid esters, phosphates and other organic defoamers were studied and applied earlier and belong to the first generation of defoamers. They have the advantages of easy raw materials, high environmental performance and low production cost; the disadvantages are low defoaming efficiency, strong specificity and harsh use conditions.
Polyether defoamers are the second generation of defoamers, mainly including linear polyethers, polyethers with alcohol or ammonia as initiators, and end-esterified polyether derivatives. The biggest advantage of polyether defoamers is their strong antifoaming ability. In addition, some polyether defoamers have excellent properties such as high temperature resistance, strong acid and alkali resistance; the disadvantages are that the use conditions are limited by temperature, the use field is narrow, the defoaming ability is poor, and the foam breaking rate is low.
Silicone defoamers (third generation defoamers) have strong defoaming performance, fast foam breaking ability, low volatility, no toxicity to the environment, no physiological inertness, and a wide range of use. Therefore, they have broad application prospects and huge market potential, but their antifoaming performance is poor.
Polyether-modified polysiloxane defoamers have the advantages of both polyether defoamers and silicone defoamers, and are the development direction of defoamers. Sometimes they can be reused according to their reverse solubility, but at present, there are few types of such defoamers, and they are still in the research and development stage, and the production cost is high.
The selection of defoamers should meet the following points:
1. Insoluble or poorly soluble in the foaming liquid
In order to break the foam, the defoamer should be concentrated and concentrated on the foam film. For the case of defoamers, it should be concentrated and concentrated instantly, and for the case of foam suppression, it should always be kept in this state. Therefore, the defoamer is in a supersaturated state in the foaming liquid, and only when it is insoluble or poorly soluble can it easily reach a supersaturated state. Insoluble or poorly soluble, it is easy to gather at the gas-liquid interface, easy to concentrate on the foam film, and can play a role at a lower concentration. For defoamers used in water systems, the molecules of the active ingredients must be strongly hydrophobic and weakly hydrophilic, and the HLB value must be in the range of 1.5-3 to work best.
2. The surface tension is lower than that of the foaming liquid
Only when the intermolecular force of the defoamer is small and the surface tension is lower than that of the foaming liquid, can the defoamer particles be immersed and expanded on the foam film. It is worth noting that the surface tension of the foaming liquid is not the surface tension of the solution, but the surface tension of the foaming solution.
3. A certain degree of affinity with the foaming liquid
Since the defoaming process is actually a competition between the speed of foam collapse and the speed of foam generation, the defoamer must be able to disperse quickly in the foaming liquid so that it can quickly play a role in a wider range of the foaming liquid. To make the defoamer diffuse faster, 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 close to the foaming liquid, they will dissolve; if they are too distant, they will be difficult to disperse. Only when the affinity is appropriate, the effect will be good.
4. No chemical reaction with the foaming liquid
When the defoamer reacts with the foaming liquid, on the one hand, the defoamer will lose its effect, and on the other hand, harmful substances may be produced, affecting the growth of microorganisms.
5. Low volatility and long action time
First, it is necessary to determine the system in which the defoamer needs to be used, whether it is a water-based system or an oil-based system. For example, in the fermentation industry, oil-based defoamers such as polyether-modified silicone or polyether should be used. The water-based coating industry should use water-based defoamers and silicone defoamers. Select the defoamer, compare the addition amount, and refer to the price to get the most suitable and economical defoamer product.
1. Dispersibility of defoamers in solution
The dispersion state and surface properties of defoamers in solution significantly affect other defoaming properties. Defoamers should have a suitable degree of dispersion and particles with too large or too small particle size will affect their defoaming activity.
2. Compatibility of defoamers in foam systems
When the surfactant is completely dissolved in the aqueous solution, it is usually oriented at the gas-liquid interface of the foam to stabilize the foam. When the surfactant is in an insoluble state or supersaturated state, the particles are dispersed in the solution and accumulate on the foam, and the foam acts as a defoamer.
3. Foaming system ambient temperature
The temperature of the foaming liquid will also affect the performance of the defoamer. When the temperature of the foaming liquid itself is relatively high, it is recommended to use a special high-temperature resistant defoamer, because if an ordinary defoamer is used, the defoaming effect will definitely be greatly reduced, and the defoamer will directly demulsify the emulsion.
4. Packaging, storage and transportation
The defoamer is 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 the commonly used chemical storage method, ensure that it is sealed after use to avoid deterioration.
5. Defoamer addition ratio
The addition of the original solution and the addition after dilution have some deviations to a certain extent, and the proportions are not equal. Due to the low concentration of surfactants, the diluted defoamer emulsion is extremely unstable and will not stratify soon. The defoaming performance is relatively poor and is not suitable for long-term storage. It is recommended to use it immediately after dilution.
The proportion of adding defoamer needs to be verified by field tests to verify the effect, and it should not be added excessively.