Views: 0 Author: Site Editor Publish Time: 2022-04-18 Origin: Site
The foam problem in water treatment has troubled a lot of people, debugging initial foam, surfactant foam, impact foam, peroxide foam, foam generated by adding non-oxidative fungicides in circulating water treatment, etc. The use of foaming agents is relatively common. This article comprehensively introduces the principle, classification, type selection and dosage of defoaming agents!
1 How to remove bubbles
1. Physical method
The methods of removing foam from a physical point of view mainly include placing baffles or screens, mechanical stirring, static electricity, freezing, heating, steam, radiation, high-speed centrifugation, pressure and decompression, high-frequency vibration, transient discharge and ultrasonic (acoustic These methods all promote the gas permeation rate at both ends of the liquid film and the drainage of the bubble film to different degrees, so that the stability factor of the foam is smaller than the attenuation factor, and the number of bubbles is gradually reduced. However, the common disadvantages of these methods are that the use is strongly restricted by environmental factors, and the defoaming rate is not high. The advantages are environmental protection and high recycling rate.
2. Chemical method
The methods of eliminating foam from a chemical point of view mainly include chemical reaction methods and methods of adding defoaming agents.
The chemical reaction method refers to the chemical reaction with the foaming agent by adding some reagents to generate water-insoluble substances, thereby reducing the concentration of the surfactant in the liquid film and promoting the rupture of the foam, but this method has foaming. There are some disadvantages, such as uncertainty of the composition of the agent, and the generation of insoluble substances that are harmful to the system equipment. Nowadays, the most widely used defoaming method in all walks of life is the method of adding defoamer. The biggest advantage of this method is that it has high foam breaking efficiency and convenient use. However, finding a suitable and efficient defoamer is the key.
2 The principle of defoamer
Defoamer, also known as defoamer, 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 that higher alcohol or vegetable oil is sprinkled on the foam, and when it dissolves into the foam liquid, it will significantly reduce the surface tension there. Because these substances are generally less soluble in water, the reduction in surface tension is limited to the local area of the foam, with little change in surface tension around the foam. The portion with reduced surface tension is strongly pulled around, stretched, and finally ruptured.
2. Destroy the elasticity of the membrane and cause the bubble to burst
When the defoamer is added to the foam system, it will diffuse to the gas-liquid interface, making it difficult for the surfactant with foam stabilization to restore the elasticity of the film.
3. Promote the liquid membrane drainage
The defoamer can promote the liquid film to discharge, thus causing the bubbles to burst. The rate of foam discharge can reflect the stability of the foam. Adding a substance that accelerates the foam discharge can also play a role in defoaming.
4. Adding hydrophobic solid particles can cause bubbles to burst
The hydrophobic solid particles on the surface of the bubbles will 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 assisting foaming surfactants can cause bubbles to burst
Some low-molecular-weight 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 octanol, ethanol, propanol and other alcohols, can not only reduce the surfactant concentration of the surface layer, but also dissolve into the surfactant adsorption layer, reducing the tightness between the surfactant molecules. degree, thereby weakening the stability of the foam.
6. Electrolyte disintegrates surfactant electric double layer
For the interaction of the electric double layer of the surfactant by means of foam to produce a stable foaming liquid, the addition of a common electrolyte can disintegrate the electric double layer of the surfactant and play a defoaming effect.
3 Classification of defoamer
Commonly used defoamer can be divided into silicon (resin), surfactant, paraffin and mineral oil according to different components.
1. Silicon (resin)
Silicone resin defoamer, also known as emulsion type defoamer, is used by emulsifying and dispersing silicone resin with an emulsifier (surfactant) in water and then adding it to wastewater. Silica fine powder is another silicon defoamer with better defoaming effect.
This type of defoamer is actually an emulsifier, that is, by using the dispersing effect of surfactants, the foam-forming substances are dispersed in a stable emulsified state in water, thereby avoiding foaming.
Paraffinic antifoaming agent is a defoaming agent made by emulsifying and dispersing paraffinic wax or its derivatives with an emulsifier. Its use is similar to that of surfactant-based emulsified defoaming agent.
4. Mineral oils
Mineral oil is the main defoaming ingredient. In order to improve the effect, metal soap, silicone oil, silica and other substances are sometimes used together. In addition, in order to make the mineral oil easily diffuse to the surface of the foaming liquid, or to uniformly disperse metal soaps and the like in the mineral oil, various surfactants are sometimes added.
4 Advantages and disadvantages of different types of defoamer
Mineral oils, amides, lower alcohols, fatty acids and fatty acid esters, phosphate esters and other organic defoamers have been researched and applied earlier and belong to the first generation of defoamers. Advantages such as low cost; disadvantages are low defoaming efficiency, strong specificity, and harsh use conditions.
Polyether defoamer is the second generation of defoamer, mainly including three kinds of linear polyether, polyether with alcohol or ammonia as the starting agent, and polyether derivatives with end-group esterification. The biggest advantage of polyether defoamer is its strong anti-foaming ability. In addition, some polyether defoamer have excellent properties such as high temperature resistance, strong acid and alkali resistance; the disadvantage is that the use conditions are limited by temperature and the field of use. Narrow, poor defoaming ability, low foam breaking rate, etc.
Silicone defoamer (third-generation defoamer) has the advantages of strong defoaming performance, rapid foam breaking ability, low volatility, non-toxic to the environment, no physiological inertness, and a wide range of applications, so it has a wide range of applications. It has great application prospects and huge market potential, but its anti-foaming performance is poor.
Polyether-modified polysiloxane defoamer combines the advantages of polyether defoamer and silicone defoamer, and is the development direction of defoamer. Sometimes it can also be reused according to its inverse solubility, but at present, there are few types of such defoamers, which are still in the research and development stage, and the production cost is relatively high.
5 Selection of defoamer
The selection of defoamer should meet the following points:
1. Insoluble or hardly soluble in the foaming liquid
In order to burst the foam, the defoamer should be concentrated and concentrated on the bubble film. For the case of foam breaker, it should be concentrated and concentrated in an instant, and for the case of foam suppressor, it should be kept in this state frequently. Therefore, the defoamer is in a supersaturated state in the foaming liquid, and it is easy to reach a supersaturated state only if it is insoluble or insoluble. Insoluble or insoluble, it is easy to gather at the gas-liquid interface, and it is easy to concentrate on the bubble film, so that it can function at a lower concentration. For the defoamer used in water system, the molecule of the active ingredient must be strong hydrophobic and weak hydrophilic, and the HLB value is in the range of 1.5-3, and the effect is the 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, the defoamer particles can immerse and expand 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. Has a certain degree of affinity with the foaming liquid
Since the defoaming process is actually a competition between the rate of foam collapse and the rate of foam formation, the defoamer must be able to quickly disperse in the foaming liquid in order to function rapidly over a wide range of the foaming liquid. To make the defoaming agent diffuse faster, the active ingredient of the defoaming agent must have a certain degree of affinity with the foaming liquid. If the active ingredient of the defoamer is too close to the foaming liquid, it will dissolve; if it is too sparse, it is difficult to disperse. Only when the affinity is appropriate, the effect will be good.
4. No chemical reaction with the foaming liquid
The antifoaming agent reacts with the foaming liquid. On the one hand, the antifoaming agent will lose its effect, and on the other hand, it may produce harmful substances and affect the growth of microorganisms.
5. Low volatility and long action time
First of all, it is necessary to determine the system that needs to use the defoamer, whether it is a water-based system or an oil-based system. For example, in the fermentation industry, oily defoamers, such as polyether-modified silicon or polyethers, are used. The water-based coating industry needs water-based defoamer and silicone defoamer. Select the defoamer, compare the amount added, and at the reference price, you can get the most suitable and economical defoamer product.
6 Factors that affect the use effect of defoamer
1. Dispersibility of defoamer in solution
The dispersion state and surface properties of the defoamer in solution significantly affect other defoaming properties. Defoamers should have a suitable degree of dispersion and particles that are too large or too small will affect their defoaming activity.
2. Compatibility of defoamer in foam system
When the surfactant is completely dissolved in the aqueous solution, it is usually oriented at the air-liquid interface of the foam to stabilize the foam. When the surfactant is in an insoluble or supersaturated state, the particles disperse in solution and accumulate on the foam, which acts as a defoamer.
3. Ambient temperature of foaming system
The temperature of the foaming liquid also affects 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 defoamer, because if a common defoamer is used, the defoaming effect will definitely be greatly reduced, and the defoamer will directly break the emulsion. .
4. Packaging, storage and transportation
The defoamer is suitable for storage at 5 to 35 °C, and the shelf life is generally 6 months. Do not place it near heat sources or expose it to sunlight. According to common chemical storage methods, make sure to seal after use to avoid spoilage.
5. The addition ratio of defoamer
The addition of the stock solution and the addition after dilution have some deviations to a certain extent, and the ratios are not equal. Due to the low concentration of surfactants, the diluted defoamer emulsion is extremely unstable, and will not be layered soon. The defoaming performance is relatively poor, and it is not suitable for long-term storage. It is recommended to use immediately after dilution.
The proportion of adding defoamer needs to be verified by field tests, and it should not be added excessively.
7 Dosage of defoamer
There are many types of defoaming agents, and the amount required for different types of defoaming agents is different. Here we introduce the amount of addition of six types of defoaming agents:
1. Alcohol defoamer: When using alcohol defoamer, the dosage is generally within 0.01-0.10%.
2. Oil and fat defoamer: the addition amount of oil and fat defoamer is between 0.05-2%, and the addition amount of fatty acid ester defoamer is between 0.002-0.2%.
3. Amide defoamer: The effect of amide defoamer is better, and the addition amount is generally within 0.002-0.005%.
4. Phosphoric acid defoamer: Phosphoric acid defoamer is most commonly used in fibers and lubricating oils, and the addition amount is between 0.025-0.25%.
5. Amine defoamer: Amine defoamer is mainly used in fiber processing, and the addition amount is 0.02-2%.
6. Ether-based defoamer: Ether-based defoamer is widely used in papermaking, printing, dyeing, and cleaning, and the addition amount is generally 0.025-0.25%.