1. Foreword: Marine fouling organisms, also known as marine attachment organisms, are animals, plants and microorganisms that grow on the bottom of ships and on the surface of all facilities in the sea. These creatures are generally harmful. The bilge organism is called biofouling, and the prevention of biofouling is called antifouling. Biological fouling is a biological hazard that humans encounter when they first come into contact with the ocean. The hazards of fouling organisms to ships and pipelines have long attracted people’s attention. In recent years, with the development of the marine industry and shipbuilding industry, the attached fouling of marine organisms has aroused great attention. In the vigorous growth and reproduction season of marine organisms, after only 12 months, it may be on the ship’s hull. , The seawater cooling system and other buildings are covered with sea creatures. Studies have shown that marine organisms have a significant impact on the corrosion rate of carbon steel and low-alloy steel. Attached organisms make the valve unsafe and tight. The flow of cooling water through the seawater pipeline and the cooler decreases, and the heat exchange efficiency of the cooler decreases, thereby increasing fuel consumption. Sometimes the ship has to stop for cleaning work, sometimes the equipment and instruments are damaged, and the rotating parts fail. , Causing serious accidents and bringing huge economic losses to people.
2. Anti-fouling treatment technology
After scientific research, people have summarized the following dozens of methods to prevent marine organisms from attaching. Although they all have certain limitations, they are all effective and counties have varying degrees of value.
1. Apply antifouling paint
Antifouling paint is composed of poisonous materials, pigments, paints, solvents, additives, etc. The poisonous materials are the most important components. The commonly used poisonous materials are toxic mercury compounds. Other commonly used poisonous materials include DDT and various organotin compounds. Its function is to rely on the toxic material to constantly seep out from the paint film, forming a toxic thin layer on the surface of the structure, repelling or killing the spores or larvae of marine creatures trying to stay on the paint film. Copper ions, mercury ions, etc. have the effect of coagulating proteins in the organism, which can achieve the purpose of antifouling.
2. Add a large amount of toxicology to seawater
The poisonous material is mainly liquid chlorine, bleaching powder and so on. Utilizing the strong oxidizing properties of these substances, the organic substances are oxidized, and the marine organisms are destroyed and killed.
3. Electrolysis of seawater to generate hypochlorite
Sea water contains a lot of chloride ions, and special electrodes are used for direct current. The seawater is electrolyzed to generate sodium hypochlorite. The low concentration of sodium hypochlorite in seawater can destroy the cell tissue of marine organisms, causing larvae, spores and spores to die or lose their ability to attach.
4. Electrolytic heavy metal method
Many heavy metals are poisonous. At present, the most widely used method of electrolytic copper or alloy is to install copper anode in sea water and dissolve copper ions into sea water through direct current electrolysis. Copper ion is a poisonous material, which can reduce the adhesion of many animal objects and play a role in preventing pollution.
5. Manual or mechanical removal method
This is a measure taken for structures that have been attached to organisms, usually during normal shutdown periods, for non-operational manual or mechanical removal. The main target is invertebrates. The use of this method to solve the problem of fouling still accounts for a large proportion .
6. Filtering method
This method is to dig deep seawater wells at the seashore, and use the filtering effect of soil and gravel to filter out the eggs, spores, larvae, etc. of sea creatures, avoiding the growth of sea creatures in the seawater transportation system.
7. Heating method
It is to feed hot water (or hot alkaline water) into the seawater transportation system to which marine organisms have been attached, and last for half an hour when the temperature _ reaches 50°C. You can kill the attachments, and then use a lot of sea water to clean the pipeline to remove the debris.
8. Closed method
Block the ends of the pipeline with organisms, the sea creatures in the pipeline will die after a few days due to lack of oxygen and bait, and then the pipeline will be flushed. Clear the wreckage.
9. Use fresh water
Due to changes in the living environment, sea creatures die on their own. Some ships sail back and forth between the ocean and the river, and sea creatures or freshwater creatures die. But its debris is accumulated in the pipeline and must be cleaned up in time.
10. Use anti-fouling lining to make structures
According to the performance requirements of the structure, select the appropriate metal or alloy to make the structure. Use the toxicity of the structure itself to hinder the attachment of fouling organisms. For example, anti-fouling copper alloys are used to make the fouling grid of the water intake head. The toxic metals include silver, arsenic, copper, lead, tin, cadmium, nickel, zinc, mercury, diamonds, etc.
11.Improve the movement speed of submerged objects
The static sea immersion is the most severely fouled. Experiments have shown that the seawater flow rate on the surface of the structure is 2.9-8.5 nautical miles/h, and all animal fouling is affected to a certain extent. When the speed reaches 11 nautical miles/h, the algae The adhesion is affected.
12. Smelly ammonia method
In recent years, with the advanced development of chemical resistance treatment, an ozone-based cooling water treatment technology with newer concepts is emerging abroad. It has been found that passing eight ozone in the cooling water can not only eliminate bacteria and viruses in the water, but also inhibit or eliminate scaling. Control equipment corrosion, no need to add any chemicals, no PH value adjustment, no pollution, no pollution to the environment, and low operating costs. This has attracted more and more people’s attention.
Compared with chlorine. In addition to having a stronger broad-spectrum imaging effect, ozone also has a faster killing rate than chlorine. The research results show that the sterilization speed of ozone is 3125 times faster than that of chlorine, that is, if the appropriate ozone concentration is selected, the killing speed of ozone is measured in seconds, and the killing speed of chlorine is measured in hours. Compared with chlorine, ozone not only has a strong ability to oxidize and destroy biological enzymes, but also diffuses into the cell wall faster than chlorine. Therefore, nature has greater lethality than chlorine. Practice has proved that the use of ozone to treat waste industry water does not need to be supplemented with non-oxidizing biocides. It also avoids the worry of environmental pollution.
This shows that the ozone method is not only technically feasible, but also very economical. But it needs to be pointed out. The ozone treatment method is still in the development stage, and its mechanism of action is not very clear. The technology still needs to be further improved, and it is difficult to predict whether it can replace the chemical method, but as a unique cooling water treatment method, it is still viable and should be developed and utilized in accordance with the specific conditions of our country.
13. Other anti-fouling methods
Ultrasonic anti-fouling uses an electronic oscillator to drive the acoustic emission device, which creates an environment where attachments are difficult to live; ultraviolet anti-fouling uses 2537×10-10m ultraviolet light to change the chemical bonds of certain molecules, and switch the light energy emission device for a long time or periodically. It can achieve long-term complete antifouling effect; there are also use of poisonous rubber layer or peelable plastic film layer, etc. These methods are limited to some local and specific environments, and their application range is very limited. .
3. Comparison of anti-pollution methods
Since the attached organisms to be prevented are diverse, and the environmental conditions, marine structures or seawater flowing through the equipment are very different, any anti-fouling method has certain limitations, the following are the most Common methods are used to make some comparisons.
1. Antifouling paint
This is an antifouling method with the widest application range and does not require management. With the continuous development of science and technology, there are more and more varieties of antifouling coatings, and the antifouling period has also been lengthened. For structures that are convenient for construction and can be repainted, this method is the most suitable, and the investment is small at a time, and coatings are almost all applied to the hull antifouling.
The limitations are: ① Short anti-fouling life; the longest anti-fouling life for one application is 3-5 years. The cost of repeated painting is high. ② The scope of anti-fouling is limited; it is limited to the painted area. ③ Difficulty in construction: The construction personnel are exposed to toxic substances at work, especially in the pipeline construction process, the conditions are extremely poor, which is harmful to health. For some thinner pipes or thick pipes that have been repainted after being put into use, it is difficult or impossible to carry out the construction. ④ Local environmental pollution may be caused; especially in fixed structures in the sea or at fixed-point seawater discharges of large pipelines, because poisonous materials are released at fixed points throughout the year, they will cause local seawater pollution. ⑤ Toxic materials have a certain waste: in some areas, No sea creatures attach in winter-because the release rate of the poisonous material in the paint film cannot be controlled artificially, the poisonous material is still continuously released in the unnecessary season. There are some international restrictions on the use of antifouling paint, such as organotin. Anti-fouling paint is forbidden to be used on yachts and fishing net cages, and it is also restricted to be painted on ocean freighters and underwater structures.
2. Apply liquid chlorine
This is also an antifouling method that has been widely used. It can be used seasonally and continuously in large doses, which is suitable for pipeline antifouling. The anti-fouling range is wide, and it can ensure that no sea creatures grow in the seawater flowing through. This method has the following shortcomings: ① Unsafe: The storage and transportation of liquid chlorine is dangerous. ② Daily management is complicated; more management personnel are required for the storage, transportation, and release of liquid chlorine into seawater. ③ One-time investment is large: need to establish a chlorination workshop, a chlorine storage workshop + purchase a chlorine storage machine, a liquid chlorine storage tank, etc. ④High operating cost: The annual cost for purchasing, transporting, and storing liquid chlorine is considerable. ⑤ The poisonous effect of liquid chlorine: Because it is easy to leak, it will cause local air pollution and endanger the health of operators. Due to the above reasons, At present, this method is being replaced by the gradual improvement of the electrolytic seawater chlorine production method.
3. Electrolysis of sea water to produce chlorine
Electrolytic seawater antifouling is the electrolysis of seawater through special electrodes. The chlorine and hypochlorite generated are strong oxidants, which are toxic to sea creatures and can kill sea creatures, and achieve the purpose of preventing sea creatures from attaching and growing. During the electrolysis of seawater, the following reactions are mainly involved:
Anodic reaction: 2Cl-→Cl2+2e (1)
40H- → O2+ 2H2O+ 4e (2)
Cathodic reaction: 2H2O+2e→2OH-+H2 (3)
The yin and yang products are mixed to generate sodium hypochlorite;
C12 + 2NaOH→NaOCl+ NaCl+H2O (4)
This is the technology first studied in the United Kingdom and Japan since the 1960s. Now it has been widely used in the United Kingdom, the United States, Japan, Canada, Italy, Russia and other countries. The scope of application is mainly seawater pipelines and seawater cooling systems. The characteristics are: ① Safe and reliable: the electrolysis of seawater is carried out in a closed pipeline, which has no effect on the health of the operators. ②Convenient daily management: simple operation, reasonable design, no personnel on duty can be used, regular inspections are used, and the device will be repaired once in winter when it is out of service. ③Economy: the raw materials used are inexhaustible seawater, and one investment is effective for a long time. Daily production only needs to spend some electricity, and the electrode plates are replaced regularly within 3-5 years; the equipment can be used economically and rationally according to the seasonal changes and the attachment of sea creatures, without causing waste. ① Wide range of anti-fouling: same as the chlorination method, the entire seawater system can be fully anti-fouling. ⑤Environmental pollution-free: Control the concentration of available chlorine in the discharged seawater to be very low, and will not cause harm to the environment and fish. The disadvantages are: ① Large one-time investment: It is necessary to establish a chlorine production workshop and purchase chlorine production equipment. ② Consume a certain amount of electricity. ③Compared with anti-fouling paint, certain management personnel are still required, and the equipment needs regular maintenance.
4. Electrolysis of heavy metals
This method has been widely used in the antifouling of seawater pipelines such as merchant ships and offshore platforms all over the world. Compared with the application of liquid chlorine method and electrolytic seawater method, the method is less investment, convenient for management, low operating cost, and wide range of antifouling. The installation is simple and the electricity used is very small. Because the anodes used are consumable, they must be replaced once within a few years (generally designed to be the same as the ship’s maintenance period). Sometimes it is inevitable to use underwater operations to replace the exhausted anodes and use this method to prevent pollution. Cause a little trouble. There are no reports of applying this method in projects that use large amounts of seawater. This is probably because the amount of copper is considerable after all, and long-term discharge at fixed points must also consider environmental pollution, and it is difficult to replace anodes.
5. Other methods
Manual or mechanical cleaning, heating, sealing, and the use of fresh water are all applied to varying degrees, mainly to solve the blockage of large seawater pipelines, and have little effect on solving the heat transfer effect of the heat exchanger.
Four, electrolytic sea water anti-fouling
At present, the three methods of electrolysis of seawater, copper anodes, and aluminum anodes are more convenient and feasible for the antifouling of seawater cooling systems of ships and coastal power plants. They have been widely used in many countries: Japan has hundreds of MGPSs (preventing sea Biological growth devices) are used on ships; the Royal Navy has been using electrolytic seawater antifouling devices on ships since 1965; American seawater electrolytic sodium hypochlorite generators are widely used in large ships; Japan’s Mitsubishi Gulf oil field, Germany’s Reeder Forshufig, Canadian Bowdril, Italian SNAM Progetti, and American Amaco have all applied copper anode and aluminum anode antifouling and corrosion protection technologies to ships and platforms. Copper anode and aluminum anode anti-fouling and anti-corrosion devices have become anti-fouling equipment for navies and merchant ships all over the world. The chlorine-copper anode anti-fouling system was developed by a consulting company affiliated to the University of Field in the United Kingdom. It is called the BFCC system. It is believed that this system represents the modern anti-fouling technology level.”
Electrolytic antifouling includes electrolytic seawater antifouling, electrolytic copper anode, electrolytic aluminum anode, and sometimes several methods are used at the same time to produce a synergistic effect. The anodes currently used mainly include diamond-silver alloy, iron-coated lead oxide, platinum/titanium, and titanium-coated. Ruthenium dioxide, titanium coated with iridium oxide, titanium coated with platinum, ruthenium oxide, palladium oxide, etc. The specific process is: electrolyze seawater with a special anode connected to direct current, electrolyze copper anode and aluminum anode at the same time, the former produces chlorine gas, kills the larvae and spores of marine organisms, plays a role of antifouling, electrolytic copper anode generates copper ions, electrolytic aluminum anode AI(OH) is produced. AI(OH) encapsulates the released copper and passes through the protected system with the flow of seawater. It has a high viscosity and will spread out and enter the sea. Creatures are likely to live in the white seawater. In areas where the flow is slow, and when the aluminum electrode system is electrolyzed in natural seawater, it acts as a cathode to form an insoluble hydroxide coating on the surface of the white iron plate. This coating can block the diffusion of oxygen and increase the concentration polarization to increase the corrosion rate. The role of mitigation. In this way, the purpose of anti-fouling and anti-corrosion can be achieved at the same time. In the process of electrolysis of seawater, due to the chlorine overpotential of the anode, the hydrogen overpotential of the cathode, and the increase in the specific impedance of seawater and side reactions, the effective salt content changes, and due to the impurities in the seawater, the actual power consumption is higher than Several times more in theory. Therefore, when designing a practical device, the basic one is to minimize the power consumption, but also consider the precipitation of Mg(OH)2 and the influence of stray current.
When implementing the method of preventing the attachment of marine organisms, there are still some essentials to be noted: At present, there are three types of arrangement of the electrolysis cells in the electrolytic seawater electrolysis cell: plate type assembly, tubular assembly, and bipolar assembly.
The installation fork of the electrode in the electrolytic cell is divided into direct type and indirect type:
The installation of electrodes on the sea water main pipe is called direct type;
Part of the sea water flow, or adopt appropriate methods to electrolyze seawater, and inject the seawater containing very concentrated electrolysis products into the main water pipe is called the indirect method.
Although the direct method is relatively simple, it has the following disadvantages: floating objects in the seawater will cause damage to the electrode, high wiring costs for large-capacity equipment, and the influence of stray current on the surroundings. Pay attention to it when using it.
There are two ways to electrolyze seawater:
① Diaphragm type
A partition composed of asbestos, asbestos cloth, glass cloth, etc. is arranged between the anode and cathode electrodes.
② No diaphragm type
Because there is no difference in current efficiency between the diaphragm type and the non-diaphragm type, and the total efficiency will vary greatly due to the change of the seawater flow rate (retention time), so when there is a cathode product like Mg(OH)2 separated When, the diaphragm type should be used. The disadvantage of adopting this method is that the structure of the electrolytic cell is more complicated and the equipment cost is high. However, the disadvantage of using the diaphragmless type is that the by-products in the electrolytic cell are difficult to use, but it has the advantages of simple structure, low cost of equipment, and convenient maintenance.
Link to this article：Titanium electrode seawater electrolysis antifouling treatment technology
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