Application and research progress of surfactants in chemical cleaning

Abstract: The principle, application and research status of surfactant in chemical cleaning were reviewed, and the existing problems were analyzed. It is pointed out that the synthesis of biodegradable green surfactants and the development of green synthesis technology of surfactants are important directions for future development.

In the industrial production process, a variety of dirt will be produced in the equipment and pipelines of the production process, such as coke, oil dirt, scale, sediment, corrosive dirt, etc. These dirt usually lead to the failure of equipment and pipelines, the production efficiency of the device system is reduced, energy consumption is increased, and even safety accidents occur in serious cases.

In recent years, with the rapid development of new synthetic industry, new industrial dirt has emerged constantly, and its molecular structure is becoming more and more complex. In addition, the bonding mechanism and bonding form between industrial dirt and different washing objects often depend on the type of dirt and the structural composition and surface physical and chemical properties of the object to be cleaned. Due to the need of environmental protection, the biodegradability and non-toxic and harmless requirements of chemical agents are increasing, so new challenges are constantly presented to chemical cleaning technology.

Chemical cleaning is a comprehensive technology, involving the study of dirt formation and performance, the selection of cleaning agents and additives, the selection of corrosion inhibitors, cleaning technology, the development and utilization of cleaning equipment, cleaning process detection technology and waste liquid treatment. Among them, the choice of cleaning agent is a key part of the success or failure of the cleaning work, which directly affects the cleaning effect, descaling rate, corrosion rate and economic benefits of the equipment.

The cleaning agent mainly consists of three components: cleaning agent, corrosion inhibitor and surfactant. Surfactants have both hydrophilic and hydrophobic parts in their molecular structure, which play the roles of adsorption, penetration, emulsification, dissolution and washing in chemical cleaning. They are not only used as auxiliary agents in chemical cleaning, but also can be used as the main component and have been widely valued. Especially in pickling, alkali washing, corrosion inhibition, oil removal, killing and other cleaning process has played a more and more important role.

A lot of research has been done on the application of surfactants in chemical cleaning at home and abroad. The present situation and progress of the application of surfactants in chemical cleaning will be briefly reviewed in this paper.

1 Overview of surfactants

Surfactants are substances that can significantly reduce the surface tension of liquids and the interfacial tension of liquids. Their molecules are composed of two parts: a polar hydrophilic group and a non-polar hydrophobic group.

The hydrophilic part is the carboxyl group, sulfate, sulfonic acid group, amine group, quaternary ammonium group, hydroxyl group, etc. The hydrophobic part is the long chain alkane and aromatic group. Because they have both hydrophilic and lipophilic amphiphilic properties, the surface properties of the cleaning solution can be changed during chemical cleaning, so that it has lower surface tension, better wettability and foaming property.

Surfactants can be divided into anionic, cationic, non-ionic and amphoteric types according to the different charges carried by the surface active part.

2 Mechanism of action of surfactants in chemical cleaning agents

Cleaning agent, corrosion inhibitor and surfactant are the three major components of chemical cleaning solution. The special chemical structure of surfactant determines that after the surfactant is dissolved in the liquid solution, the surface tension of the solution is greatly reduced and the wetting ability of the solution is improved. Especially when the concentration of surfactant in the solution reaches the critical micellar CMC concentration, the surface tension, osmotic pressure, viscosity and optical properties of the solution have significant changes.

The wetting, permeation, dispersion, emulsification and solubilization of surfactants in the chemical cleaning process have the effect of twice the result with half the effort. In summary, there are two main functions of surfactants in chemical cleaning. One is to increase the apparent concentration of insoluble organic pollutants by the dissolution of micelles, that is, solubilization; Second, because the surfactant has amphiphilic groups, it can adsorb or enrich on the interface of oil phase and water phase, so that the interfacial tension is reduced.

3 Application of surfactants in chemical cleaning agents

In the selection of surfactants, special attention should be paid to the properties of cleaning agents, corrosion inhibitors, surfactants and the matching of their interactions.

3.1 Application in corrosion inhibitors

Surfactants are additives that can have a significant effect on the corrosion inhibition process. They can increase the wettability, dispersion and foamability, promote the contact of the pickling solution with scale and rust, and change the surface state of the matrix metal after pickling, so as to improve the quality of pickling. Some surfactants also have certain corrosion inhibition properties, which can get better results than using a single corrosion inhibition component. The surfactants generally added in the corrosion inhibitor formulation are anionic C10~C18 alkyl or alkyl benzene sulfonate or alkyl sulfate, or non-ionic higher alcohols, phenolic polyoxyethylene compounds. However, some corrosion inhibitors also have cationic surfactants, such as quaternary ammonium salts.

It has been reported that the non-ionic surfactant Tween80 has a good corrosion inhibition effect on steel in three inorganic acid media (HCl, H2SO4, H3PO4). Yuan Langbai et al. studied the synergistic effect of anionic surfactant sodium dodecyl benzene sulfonate (DBSAS) and non-ionic surfactant polyethylene glycol octylphenyl ether (OP) on steel corrosion inhibition, and found that the combination of DBSAS and OP produced obvious synergistic effect on steel corrosion inhibition, and the concentration range of the combination was wide.

3.2 Application in pickling

3.2.1 Used as an acid fog inhibitor

In pickling, hydrochloric acid, sulfuric acid or nitric acid, while reacting with rust and scale, will inevitably react with metal matrix, release heat, and produce a large amount of acid fog. Due to the action of hydrophobic group, an insoluble linear film covering layer is formed on the surface of the pickling solution by adding surfactant to the pickling solution, and the volatilization of acid mist can be inhibited by using the foaming action of surfactant. Of course, corrosion inhibitors are often added to the general pickling solution, which can greatly reduce the corrosion rate of the metal and reduce the amount of hydrogen evolution, that is, the acid fog is correspondingly reduced.

Wang Ruifeng et al. developed BSY type acid fog inhibitor, which can effectively inhibit acid fog in acid-base treatment of metal materials; Yuan Jiaoqiu of Changsha Research Institute of Mining and Metallurgy developed YJ-509 acid fog inhibitor, the fog suppression efficiency can reach more than 92%.

3.2.2 Used for pickling and oil removal two-in-one cleaning

In the chemical cleaning of general industrial equipment, if the dirt has grease components, in order to ensure the quality of pickling, it is first necessary to undergo alkali washing and then pickling. If a certain amount of degreasing agent based on non-ionic surfactant is added to the acid-washing solution, it can be combined into one process. In addition, the general solid cleaning solution is mostly sulfamic acid as the main component, and contains a certain amount of surfactants, thiourea and inorganic salts and other components, which are mixed with water when used. This cleaning agent not only has good rust, scale and corrosion inhibition performance, but also can remove oil at the same time.

The BH-6 high-efficiency pickling and oil removal agent developed by Liang Guozhu and others has a remarkable effect, and has been applied in more than 100 domestic electroplating and spraying plants in recent years, and has achieved good economic and social benefits; Yu Cunye et al. used the cleaning solution of compound inorganic acid plus non-ionic surfactant and corrosion inhibitor for spray cycle cleaning, and achieved satisfactory results.

3.3 Application in alkali washing

3.3.1 General Device cleaning

Alkali washing is a kind of cleaning method that uses a strong alkaline chemical agent as a cleaning agent to loosen, emulsify and disperse the dirt in metal equipment. It is often used as a pre-treatment for pickling to remove the grease in the system and equipment or transform the insoluble scale such as sulfate and silicate, so that pickling is easy to carry out. Commonly used alkali washing agents are sodium hydroxide, sodium carbonate, sodium phosphate or sodium silicate, while adding surfactants in order to moisten grease and disperse dirt, improve the effect of alkali washing.

Xu Gaoyang et al. found that adding the additive NPE-10 to the heavy alkali washing water can effectively reduce the heavy alkali water content and NaCl content, which not only reduces the number of shutdown times, but also improves the production capacity of the combustion furnace. The oil removal formula of the equipment before pickling studied by Yu Cunye is as follows: sodium hydroxide 3%~5%, sodium carbonate 2%~3%, sodium tripolyphosphate 4%~5%, water glass 1%~1.5%, sodium dodecyl sulfonate 2%~5%, 80~90℃.

3.3.2 Used as water-based metal cleaning agent

Water-based metal cleaning agent is a kind of detergent which takes surfactant as solute, water as solvent and metal hard surface as cleaning object. It can replace gasoline and kerosene to save energy, and is mainly used for metal cleaning in machinery manufacturing and repair, machinery and equipment repair and maintenance. Sometimes it can also be used as a general oil dirt cleaning of petrochemical equipment. The water-based cleaning agent is mainly composed of a mixture of nonionic surfactant and anionic surfactant, and a variety of additives. The former has large cleaning power and good rust prevention and corrosion inhibition ability, while the latter can improve and improve the comprehensive performance of cleaning agent.

In the selection of surfactants, special attention should be paid to the properties of cleaning agents, corrosion inhibitors, surfactants and the matching of their interactions.

3.4 Application in complex cleaning

Complexing agent is also known as chelating agent or ligand, it is the use of various complexing agent (including chelating agent) on various scale ions complexation (coordination) or chelation, so that it generates soluble complex (coordination compound) and cleaning.

Surfactants are often added to the cleaning of complexing agents to facilitate the cleaning process. The commonly used inorganic complexing agents are sodium tripolyphosphate, and the commonly used organic chelating agents are ethylenediamine tetraacetic acid (EDTA), nitrogen triacetic acid (NTA), etc. In addition to the cleaning of the cooling water system, the cleaning of the complex agent has been greatly developed in the cleaning of insoluble scale. Because it can complex or chelate metal ions in various insoluble scale, it has high cleaning efficiency.

The solution chamber of bromine machine was cleaned and pre-coated by alkali complex cleaning method by Huang Xinfa et al. in sinter plant of WISCO, which achieved the purpose of reducing corrosion and prolonging service life of bromine machine.

3.5 Application in cleaning heavy oil scale and coke scale

In petroleum refining and petrochemical equipment, heavy oil and coke scale deposits in heat exchange equipment and pipelines are serious, and they often need to be cleaned. The use of organic solvents is highly toxic, flammable and explosive; However, the general alkaline washing method is ineffective for heavy oil and burnt dirt.

At present, the heavy oil cleaning agents developed at home and abroad are mainly compound surfactants, which are composed of several non-ionic surfactants and anionic surfactants, and inorganic detergent and alkaline substances. The composite surfactant not only produces wetting, penetration, emulsification, dispersion, solubilization and foaming effects, but also has the effect of absorbing FeS2, which generally needs to be heated and cleaned above 80℃.

Wang Guotai et al., using surfactant, organic solvent and water as raw materials, developed a kind of hard surface oil scale cleaning agent with excellent performance by using the phase-shifting emulsification method, which can efficiently and quickly remove the oily dirt on various hard surfaces (such as kitchen stoves, glass, porcelain, etc.), especially the oil scale and scorch scale on various metal surfaces.

3.6 Application of cooling water biocides

When there are microbial slime in the cooling water system, using non-oxidizing biocides and adding low-bubble non-ionic surfactants as dispersants and penetrants can increase the activity of the agents, and promote the agents to penetrate into the cells and the mucous layer of fungi.

In addition, quaternary amine salt biocides are also widely used, which is a number of cationic surfactants, the most commonly used are Germinate and new Germinate, which have the advantages of strong killing force, easy to use, low toxicity and low cost. In addition to the function of stripping mud and removing odor in water, they also have the effect of corrosion inhibition.

In addition, the compound biocide of quaternary amine salt and dithiocyanomethane not only has a broad spectrum and synergistic biocide effect, but also can prevent the growth of slime.

4. Environmental hazards of surfactants

Surfactants are widely used in various sectors of industrial and agricultural production because of their unique properties, especially in the cleaning industry plays a pivotal role. However, a large number of surfactants enter the ecological environment through various ways, and the harm to the environment is becoming more and more serious. Most of the surfactants are discharged into rivers, oceans and other natural water bodies without proper treatment, which seriously affects the balanced development of the surrounding ecosystem.

Surfactants produce a large amount of foam at low concentration, which not only affects the beauty of natural water landscape, but also directly threatens the survival of aquatic animals and plants with its biological toxicity. In addition, due to the existence of surfactants in water, the aerobic biodegradation consumes a lot of oxygen, which significantly reduces the dissolved oxygen in water, resulting in the deterioration of water quality. Some surfactants not only inhibit and kill microorganisms, but also inhibit the degradation of other toxic substances.

On the other hand, after a large number of surfactants in water that have not been degraded are adsorbed by soil, they affect some properties of soil, significantly reduce the adsorption effect of soil on organic poisons, and change the composition of microorganisms in soil, causing further pollution. The pollution of surfactants has become a difficult problem in the environmental field. Therefore, in the process of research and application of surfactants, we should take into account the possible harm and pollution of surfactants themselves and the effects of migration, transformation and accumulation in the environment.

5 Development Prospects

With the increasing awareness of environmental protection, more and more attention has been paid to the environmental safety of surfactants, such as the discussion on whether surfactants cause harm to environmental safety, the ecology and toxicity assessment of surfactants. At the same time, many people worry that with a wide variety of daily chemical products continue to become People's Daily necessities, such as toothpaste, soap, dishwashing detergent, washing powder (agent) and all kinds of cosmetics, everyone will more or less take a certain amount of surfactants every day. Therefore, the research and development of environmentally friendly green surfactants with low toxicity and non-toxicity to the human body is getting more and more popular.

At present, the research and production of surfactants are developing in the direction of greening and functionalization. It is manifested in:

1) Improve the molecular structure of surfactants and develop green surfactants that are easy to biodegrade. For example, alkyl, aryl sulfonate, polyoxyethylene alkyl phenol ether and other difficult biodegradable surfactants such as alkyl, alkyl sulfonate and polyoxyethylene alkyl phenol ether containing benzene ring are gradually replaced by alkenyl sulfonate, oxidized amine, polyoxyethylene non-ionic and crown ether, starch, amino acid polymer surfactants, amphoteric surfactants and biosurfactants with excellent degradation performance.

2) Design and develop surfactants that are mild to the human body, so as to avoid major damage to the human skin and internal organs during the use of surfactants.

3) In the synthesis process of surfactants, make full use of the synergistic effect between surfactants, screen and develop multi-functional treatment agents, expand the cleaning function, and reduce the cost of treatment agents.

4) While considering the greening of surfactant products and raw materials, the development and research of green chemical synthesis technology of surfactants should be strengthened.

Source: IESD Surfactants and Detergents Show