Foam problems in water treatment troubled a lot of people, debugging the initial foam, surfactant foam, impact foam, peroxide foam, circulating water treatment by adding non-oxidizing biocides generated by the foam and so on, so the use of antifoam in water treatment is relatively common, this article comprehensively introduces the principle of antifoam agent, classification, selection and dosage!
Foam elimination methods
1. Physical methods
Methods of eliminating foam from a physical point of view mainly include placing baffles or screens, mechanical agitation, electrostatics, refrigeration, heating, steam, ray irradiation, high-speed centrifugation, pressurization and decompression, high-frequency vibration, instantaneous discharge and ultrasonic waves (acoustic liquid control), etc. All of these methods promote the rate of gas permeation through the two ends of the liquid film and the drainage of the bubble film to different degrees, making the stabilizing factor of the foam less than the attenuating factor and thus the number of bubbles gradually decreases. However, the common disadvantage of these methods is that the use of environmental factors by the strong constraints, defoaming rate is not high, but the advantages of environmental protection, high reuse rate.
2. Chemical methods
Methods of eliminating foam from the chemical point of view mainly include the chemical reaction method and the method of adding defoamer.
Chemical reaction method refers to the chemical reaction with the foaming agent by adding some reagents to generate insoluble substances, thereby reducing the concentration of surfactant in the liquid film, prompting the rupture of the foam, but this method exists in the foaming agent composition is not certain, the generation of insoluble substances on the system equipment to produce hazards and other shortcomings. Nowadays, the most widely used defoaming method in various industries is the method of adding defoamer, the biggest advantage of this method lies in the high efficiency of bubble breaking, easy to use and other advantages, but finding a suitable and efficient defoamer is the key.
Principle of defoamer
Defoamers, also known as defoamers, are based on the following principles:
1. Foam localized surface tension reduction leads to foam bursting
The origin of this mechanism is the sprinkling of higher alcohols or vegetable oils on the foam, which, when dissolved in the foam solution, significantly reduces the surface tension at that location. Because these substances are generally less soluble in water, the reduction in surface tension is limited to the localized area of the foam, while the surface tension around the foam remains virtually unchanged. The part with reduced surface tension is strongly pulled and extended in all directions and finally ruptures.
2、Destroy the membrane elasticity and lead to bubble bursting
The defoamer added to the foam system will diffuse to the gas-liquid interface, making it difficult for the surfactant, which has a foam stabilizing effect, to undergo the ability to restore film elasticity.
3. Promote fluid membrane drainage
Antifoam agent can promote the liquid film discharge, thus leading to bubble bursting, the rate of foam discharge can reflect the stability of the foam, add a substance to accelerate the foam discharge, can also play a role in defoaming.
4, adding hydrophobic solid particles can lead to bubble bursting
Hydrophobic solid particles on the bubble surface attract the hydrophobic end of the surfactant, causing the hydrophobic particles to become hydrophilic and enter the aqueous phase, thus acting as an antifoam.
5, solubilizing anti-foaming surfactants can lead to bubble bursting
Certain low molecular substances that can be fully mixed with the solution can make the bubble surfactant be solubilized, so that its effective concentration is reduced. This effect of low molecular substances such as octanol, ethanol, propanol and other alcohols, not only can reduce the surfactant concentration of the surface layer, but also dissolve into the surfactant adsorption layer, reducing the degree of compactness between surfactant molecules, thus weakening the stability of the bubble.
6. Electrolyte disintegration of surfactant bilayers
In the case of a foaming liquid that is stabilized by the interaction of the surfactant bilayer of the foam, the addition of a common electrolyte can disintegrate the surfactant bilayer to produce an antifoaming effect.
Classification of defoamers
Commonly used defoamers can be divided into silicon (resin), surfactants, alkane hydrocarbons and mineral oils according to the different components.
1、Silicon（Resin）type
Silicone resin antifoam agent is also known as emulsion-type antifoam agent, the use of silicone resin with emulsifier (surfactant) emulsification dispersed in water and then added to the wastewater. Silicon dioxide fine powder is another silicone antifoam agent with better defoaming effect.
2、Surfactants
This type of antifoam agent is actually an emulsifier, i.e., using the dispersing effect of surfactants to keep the foam-forming substances dispersed in the water in a stable emulsified state, thus avoiding the generation of foam.
3. Chain alkanes
Chain alkane antifoam agent is an antifoam agent made by emulsifying and dispersing chain alkane waxes or their derivatives with an emulsifier, and its use is similar to that of the emulsifying antifoam agent of surfactant class.
4. Mineral oils
Mineral oil is the main antifoaming ingredient. In order to improve the effect, it is sometimes mixed with metal soap, silicone oil, silicon dioxide and other substances and used together. In addition, in order to make the mineral oil easily spread to the surface of the foaming liquid, or to make the metal soap, etc. uniformly dispersed in the mineral oil, sometimes can be injected with a variety of surfactants.
Advantages and disadvantages of different types of defoamers
Mineral oils, amides, lower alcohols, fatty acids and fatty acid esters, phosphate esters and other organic antifoam agent research and application of early, belonging to the first generation of antifoam agent, which has the advantages of easy to obtain raw materials, high environmental performance, low production costs, etc.; the disadvantage of defoamer efficiency, specialization, the use of harsh conditions.
Polyether antifoam agent is the second generation of antifoam agent, mainly including straight chain polyether, alcohol or ammonia as the initiator of the polyether, end group esterification of polyether derivatives of three. The biggest advantage of polyether antifoam agent is that it has strong anti-foaming ability, besides, some polyether antifoam agents have excellent performance such as high temperature resistance, resistance to strong acid and alkali, etc.; the disadvantage is that the conditions of use are limited by the temperature, the field of use is narrow, the anti-foaming ability is poor, and the rate of breaking the bubble is low.
Organosilicon antifoam agent (the third generation of antifoam agent) has the advantages of strong defoaming performance, fast foam breaking ability, low volatility, non-toxic to the environment, non-physiological inertia, wide range of advantages, and so on, so it has a broad application prospects and huge market potential, but the foam inhibition performance is poor.
Polyether modified polysiloxane antifoam agent has the advantages of both polyether antifoam agent and organosilicon antifoam agent, which is the development direction of antifoam agent. Sometimes it can also be reused according to its reverse solubility, but at present the types of such defoamers are fewer, still in the research and development stage, and the cost of production is higher.
Selection of defoamer
The choice of defoamer should comply with the following points:
1、Insoluble or insoluble in foaming solution
To break the foam, the antifoam agent should be concentrated and centralized on the foam film. For the case of foam-breaking agent, it should be concentrated and centralized in an instant, and for the case of foam suppression it should be kept in this state frequently. So the antifoam agent is supersaturated in the foaming liquid, only insoluble or insoluble is easy to reach the supersaturated state. Insoluble or insoluble, it is easy to gather in the gas-liquid interface, it is easy to concentrate in the bubble film, in order to play a role in the lower concentration. Antifoam agent for the water system, the molecules of the active ingredient must be strong hydrophobic and weak hydrophilic, HLB value in the range of 1.5-3, the role of the best.
2、Surface tension is lower than foaming liquid
Only when the intermolecular force of antifoam agent is small and the surface tension is lower than that of the foaming solution, the antifoam agent particles can be immersed and expanded on the bubble 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 anti-foaming solution.
3、与起泡液有一定程度的亲和性
由于消泡过程实际上是泡沫崩溃速度与泡沫生成速度的竞争，所以消泡剂必须能在起泡液中快速分散，以便迅速在起泡液中较广泛的范围内发挥作用。要使消泡剂扩散较快，消泡剂活性成分须与起泡液具有一定程度的亲和性。消泡剂活性成分与起泡液过亲，会溶解；过疏又难于分散。只有亲疏适宜，效力才会好。
4、与起泡液不发生化学反应
消泡剂与起泡液发生反应，一方面消泡剂会丧失作用，另一方面可能产生有害物质，影响微生物的生长。
5、挥发性小，作用时间长
首先要确定需要使用消泡剂的体系，是水性体系或油性体系。如发酵行业，就要使用油性的消泡剂，如聚醚改性硅或聚醚类的。水性涂料行业就要用水性消泡剂，有机硅消泡剂。选择出消泡剂，比较添加量，在参考价格，可得出最适用最经济的消泡剂产品。
影响消泡剂使用效果的因素
1、消泡剂在溶液中的分散性
消泡剂在溶液中的分散状态和表面性能显著影响其它消泡性能。消泡剂应具有合适的分散程度和粒径太大或太小的颗粒会影响它们的消泡活性。
2、泡沫体系中消泡剂的相容性
当表面活性剂完全溶解在水溶液中时，它通常定向排列在泡沫的气液界面上以稳定泡沫。当表面活性剂处于不溶状态或过饱和状态时，颗粒分散在溶液中并积聚在泡沫上，泡沫充当消泡剂。
3、发泡系统环境温度
发泡液体的温度也会影响消泡剂的性能。当发泡液体本身的温度相对较高时，建议必须使用特殊的耐高温消泡剂，因为如果使用普通的消泡剂，消泡效果肯定会大大降低，而且消泡剂会直接将乳液破乳。
4、包装、储存和运输
消泡剂适用于5～35℃储存，保质期一般为6个月。不要把它放在热源附近或暴露在阳光下。根据常用的化学储存方法，确保使用后密封，避免变质。
5、消泡剂的添加比例
原液的添加和稀释后的添加在一定程度上有一些偏差，并且比例不相等。由于表面活性剂浓度低，稀释的消泡剂乳液极不稳定，很快就不会分层消泡性能相对较差，不适合长期储存建议稀释后立即使用。
添加消泡剂的比例需要通过现场试验来验证效果，不应该过分添加。
消泡剂的用量
消泡剂的种类很多，不同类型的消泡剂所需要的添加量是不同的，下面我们为大家介绍一下六类消泡剂的添加量：
1、醇类消泡剂：醇类消泡剂使用时，用量一般在0.01-0.10%以内。
2、油脂类消泡剂：油脂类消泡剂的添加量在0.05-2%之间，脂肪酸酯类消泡剂的添加量是0.002-0.2%之间。
3、酰胺类消泡剂：酰胺类消泡剂效果比较好，添加量一般在0.002-0.005%以内。
4、磷酸酸类消泡剂：磷酸酸类消泡剂最常用在纤维和润滑油中，添加量为0.025-0.25%之间。
5、胺类消泡剂：胺类消泡剂主要在纤维加工中使用，添加量为0.02-2%。
6、醚类消泡剂：醚类消泡剂在造纸印染、净洗中用的比较多，添加量一般是0.025-0.25%。
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