Why is seawater desalination technology so difficult?

First of all, seawater desalination technology is relatively mature at present, mainly divided into two categories: membrane method and thermal method. Generally speaking, the membrane method is filtration, which uses the high pressure applied by the outside (high pressure pump in the device) to squeeze the fresh water in seawater to one side of the reverse osmosis membrane, leaving inorganic salts and other components on the other side of the membrane. The law of heat is distillation. Imagine that it is similar to steaming in a big pot, then pure water becomes steam and then condenses to get fresh water. Of course, in practice, the design of this pot is a little more troublesome …

Seawater desalination is very mature in the Middle East. There are many desalination plants in Israel, Saudi Arabia and other places with advanced technology. Both engineering ability and technology research and development ability are worth learning. At present, the largest membrane and thermal projects in the world are over there. The maximum scale of the membrane method reached nearly 640,000 tons/day ... and the desalinated water mixed with other water sources entered the municipal pipe network. In other words, many residents are drinking desalinated water.

Of course, there are some very good companies in Japan, South Korea and Singapore. At present, the domestic thermal method is mainly low temperature and multi-effect, and the largest one is Tianjin Beijiang Power Plant, with a capacity of 200,000 tons/day, but it is not at full capacity at present; The membrane method is reverse osmosis, and the largest one is Tianjin Dagang Xinquan (made by Singapore Kaifa), with a production capacity of 654.38+10,000 tons/day, which is not fully loaded. There are others, but in general, these two are typical large-scale desalination projects, and they are running well.

Heat energy consumption is mainly steam and electricity, and steam is very expensive. Generally speaking, thermal seawater desalination is built together with power plants, because there is a lot of waste heat available in power plants, which reduces the cost of steam. Membrane method mainly consumes electricity, after all, it needs high-pressure pump to keep rotating.

Both of them need a lot of equipment investment. Generally speaking, the cost per ton of desalinated water is 4-6 yuan. Membrane devices occupy a small area and are easy to move and operate, so they are relatively widely used in the market at present. Technically, seawater desalination is very mature. Of course, the current localization rate is similar.

Just talk about some existing technical problems.

First, the localization rate is relatively low. Ultrafiltration membrane is ok, but it is still a little behind foreign membranes in real use. As for energy recovery and reverse osmosis membranes, they are basically imported. It really takes effort.

Design of typical seawater desalination plant

The two most commonly used seawater desalination technologies are reverse osmosis (accounting for 47.2% of the global seawater desalination capacity) and multistage flash evaporation (accounting for 36.5% of the global seawater desalination capacity).

Generally speaking, if we find it easy, it is mainly because of distillation, one of the methods commonly used in high school chemistry laboratories to prepare pure water. It seems not difficult, but it is not. Business is a bit troublesome.

Reverse osmosis water is the cleanest water treatment at present, and there are no minerals and metals in the water except water molecules. Poor conductivity. Multi-stage flash evaporation is a seawater desalination method. The emphasis is on multi-stage flash evaporation, and its device consists of multiple flash evaporation chambers.

Reduce the boiling point through decompression to generate steam, and then condense the steam to obtain fresh water. Because this method does not make the brine really boil (only surface boiling) and contact with the heat transfer surface area, it can greatly improve the scaling problem caused by distillation.

It was commercialized in the 1950 s.

Why does desalination feel so difficult? The difficulty of science and technology is actually not great, but the estimated benefit is not high. It is not so easy to establish a fresh water processing base.

Seawater desalination technology has been very stable, but it is difficult to reduce the cost.

At present, the technology is mainly divided into two categories: membrane reverse osmosis and distillation, among which membrane reverse osmosis technology is more cost-effective.

In many countries in the world, such as the Middle East, seawater desalination and desalination have been widely used to provide edible water. In Southeast Asia, Singapore currently has two desalination plants with a daily processing capacity of about 500,000 tons. The third desalination plant will be put into operation at the end of this year, and the fifth desalination plant on Jurong Island is expected to produce about 900,000 tons of purified water per day in 2020, accounting for about half of Singapore's fresh water supply. The goal is that by 2060, the production of fresh water and desalinated seawater will occupy 85% of water.

It can be seen from the above that the seawater desalination technology has matured and the difficulties have been overcome. At this stage, how to reduce the cost is a problem.

Water is the source of life, an important part of organisms and the cheapest "medicine" in the world.

Water plays an important physiological role in human body, so it is very beneficial to health to supplement appropriate amount of water. The water demand of human body varies with age, weight, climate and exercise intensity.

Generally speaking, adults need to replenish 1500~2800mL of water every day to supplement the water lost by urination, breathing and sweating.

A large part of this is obtained through direct drinking water, and some is obtained through diet and metabolism. In addition to human needs, daily life and production operation are also inseparable from water. Therefore, the quality of water affects the living standard and industrial production to a great extent.

However, such an important water does not always meet human needs. The distribution of fresh water on the earth is unbalanced with the distribution of economy and population. Among them, Lake Baikal has 20% fresh water reserves on the earth's surface. Antarctica, which is covered with snow and inaccessible, has 72% fresh water reserves on the earth.

Due to various reasons, many countries or regions in the world are in a state of serious water shortage, including China, the Middle East and Africa.

In order to end the uneven distribution of fresh water in the north and south and the shortage of water resources in the north, the state has implemented a huge project of "South-to-North Water Transfer". For those countries in the Middle East that are short of water but not short of oil and money, they have the heart but are powerless, and they can only find ways to solve this thorny problem, such as transporting glaciers from the North and South Poles and desalinating seawater.

Expensive seawater desalination has little effect. As early as 400 years ago, the British royal family offered a reward for finding an economical and effective seawater desalination method.

In Europe in the16th century, some people have tried to extract fresh water from seawater to meet the demand for fresh water for long-term maritime navigation.

However, due to the backwardness of science and technology, seawater desalination can only meet the daily needs of a few people and cannot be scaled and industrialized.

Until 1950s, with the aggravation of water crisis, seawater desalination developed rapidly, and countries all over the world invested a lot of manpower and material resources to study seawater desalination technology in order to find an economical and efficient industrialization method.

The world's first desalination plant was built and put into use in Freeport, Texas, USA in 1954, and it is still providing domestic water for citizens.

Up to now, there are more than 20 methods of seawater desalination technology, including reverse osmosis, low multi-effect, multi-stage flash evaporation, electrodialysis, pressure steam distillation, dew point evaporation, hydropower cogeneration, hot film cogeneration and so on.

Generally speaking, it can be divided into distillation (thermal method) and membrane method, among which low multi-effect distillation, multi-stage flash evaporation and reverse osmosis membrane method are the mainstream technologies in the world.

Reverse osmosis membrane method has the advantages of low investment and low energy consumption, but it requires high seawater pretreatment. Multi-stage flash evaporation method is mature in technology, reliable in operation, large in plant output, but high in energy consumption.

The principle of desalination is not difficult to save money. In many people's minds, it is very simple. Indeed, the principle of seawater desalination is very simple, that is, the salt in seawater and other substances that affect direct drinking are separated by physical or chemical methods.

The simplest and most direct method is distillation. With the help of different volatilities of different substances in seawater, water is evaporated and then condensed to obtain pure water. Considering the feasibility, this method can; However, from an economic point of view, energy consumption is high.

Therefore, the reverse osmosis membrane method, which has the advantages of low investment and low energy consumption, has become the main development direction of this society based on economic benefits, and its energy consumption is only 1/40 of that of distillation method.

The reverse osmosis method uses hydrogen bond theory, preferential adsorption-capillary flow theory and other theories, involving analytical chemistry, material chemistry, fluid mechanics and other disciplines. Because it needs to be carried out under high pressure, the requirements for semi-permeable membrane materials are very high.

According to the requirements of raw water quality and effluent, the pretreatment system can adopt coarse filtration, activated carbon adsorption and fine filtration. In order to protect the reverse osmosis membrane and prolong its service life, fine filtration is essential.

In addition, the composite membrane is very sensitive to free chlorine in water, and seawater contains a lot of chlorine, so the pretreatment system is usually equipped with activated carbon adsorption.

Science and technology make life better. Compared with obtaining fresh water directly from the surface or underground, seawater desalination costs more, has a large initial investment and a long cost recovery period. However, with the technological innovation and the transformation of production mode, the cost of seawater desalination has been reduced to 4-5 yuan/ton, and the economic feasibility has been greatly improved, making the embarrassing phenomenon that "water is more expensive than oil" no longer exist in a few areas in the Middle East.

Among them, Saudi Arabia, a local tyrant in the Middle East who is not short of oil and gas, not only has money, but also has a solid foundation in seawater desalination, with a global desalination capacity of 24%. In addition, the second phase of Jebel Ali seawater desalination plant in the United Arab Emirates is the largest seawater desalination plant in the world, which can produce 300 million cubic meters of fresh water every year.

Considering the cost reduction brought by technological progress in the future and the increasing demand for fresh water brought by rapid economic and social development, desalinated seawater will enter more ordinary people's families. At the same time, the seawater desalination industry is expected to break out in the future with broad prospects.

Seawater desalination technology is not difficult, but the cost is too high!

As shown in the above figure, the principle of seawater desalination is mainly filtered by water pump, laminated filter, multi-media filter, precision filter and reverse osmosis filter, and then it can be turned into fresh water! According to the experience of the United Arab Emirates and other countries, a ton of seawater desalination needs about 4-6 yuan RMB! Although it seems that this price is similar to 5 yuan, the lowest price of Beijing residents' ladder water price! But this is the cost price of seawater desalination plant! According to the cost price of tap water is only half of the ex-factory price, the price of purified fresh water from seawater should be between 8 yuan and 12 yuan, which is basically double the existing water price!

Secondly, tap water is treated locally, so its transportation cost is not high. According to the geographical characteristics of our country, there is not much water shortage in the eastern coastal areas, but many areas in the west are very short of water. However, if the water transfer project is to be built, the cost will be high, and not only the water transfer line needs high investment (refer to the budget of more than 200 billion yuan for the South-to-North Water Transfer Project). At the same time, as can be seen from the above figure, China's coastal areas are all plains with low altitude, while the western regions are all plateaus. To pump water to the west, the cost will increase greatly! It is estimated that the cost of such desalinated water is expected to be at least in 20 yuan/ton, or even more!

In a word, seawater desalination technology is very mature, but the desalination process needs high cost. In view of the topographical features of China, the transportation cost can be said to be extremely high. So desalination can't be the main source of our fresh water! With the continuous evolution of technology, it is believed that seawater desalination will become more simple, efficient and practical.

Although seawater contains many minerals, it is difficult to separate one of them. But now, a group of scientists from Australia and the United States have developed a new seawater desalination technology, which can not only make the seawater produced by this technology drinkable, but also collect lithium ions that can be used for battery production.

The key of this technology is the metal-organic framework (MOFs), which has the largest internal surface area of any known material. Theoretically, such a material can cover a football field after one gram of light is unfolded, and its complex internal structure makes MOFs a perfect object for capturing, storing and releasing molecules. Recent studies have found that this material can make MOFs find applications in carbon emission sponges, high-precision chemical sensors and urban water filters.

At present, the most commonly used water filtration technology is reverse osmosis membrane, and its principle is quite simple: the pores of the membrane can let water molecules pass through but not most pollutants. However, a big problem with this technology is that it requires a relatively high pressure to transmit water pressure.

However, MOF membrane has stronger selectivity and efficiency. Researchers from Monash University, CSIRO and the University of Texas at Austin have developed such a film. This design is inspired by the "ion selectivity" of biological cell membrane, which allows specific ions to pass through. In addition, this kind of filtration membrane does not need strong external force like reverse osmosis membrane.

Besides clean drinking water, MOF membrane can also collect lithium ions. Because the global demand for lithium in electronics and batteries is very large, and seawater is rich in lithium ions, the birth of MOF thin films is good news.

In addition, this technology will also be applied to the filtration of industrial wastewater.

The study was published in the journal Science Progress.

Theoretically, the technology of separating salt from fresh water by seawater is really simple. For example, the coastal salt drying field and seawater distillation technology are simple physical technologies, but the difficulty of seawater desalination lies not in the separation technology, but in the fact that the cost of separation and collection is not proportional to the return. Let's take seawater as an example. Basically, the salt drying method of seawater all over the world is to introduce a large amount of seawater into the salt field, directly pass through the high-temperature direct sunlight, and the crystal left after seawater evaporation is salt, and then through the related purification technology, the glistening salt is obtained. In this process, because when seawater evaporates, salt molecules are larger and will not evaporate at the same time as water molecules, so as long as there is enough temperature, salt and water can be separated, and temperature is the only requirement. From the technical point of view of drying salt, it seems not difficult to separate salt from water, but in the process of drying salt, the sun shines directly and salt is collected instead of fresh water, which is very different. If you put a piece of glass above the salt drying field, you can also get condensed fresh water droplets, but the quantity is very limited, which can not alleviate the earth's demand for fresh water at all. Moreover, the total amount of fresh water used by human beings is much higher than that of salt, which requires the evaporation temperature of seawater to be higher and wider. The equipment invented by human beings is put into operation, but there is a cost if there is equipment. In general coastal areas, the construction of several small desalination plants can basically meet the living and production needs of surrounding areas, while more than 90% of the world's regions are short of water, especially inland areas. The cost of seawater desalination equipment itself is very high. If the seawater is transported inland, the transportation cost will be higher. To put it bluntly, under the condition that water resources are exchanged with other resources on the earth, the loss to other resources is too great, and the loss is not proportional to the gain. Seawater desalination will never become an important source of fresh water until more economical equipment is invented. Welcome to pay attention to the "geographically interesting" message and discuss it together.

Our company has done a large-scale desalination plant project in North Africa. The design and reverse osmosis membrane are made by Singaporeans, and our company does the construction.

Seawater desalination technology is now very mature, mainly including reverse osmosis membrane method and thermal evaporation method.

Because many inorganic salts in seawater cannot be precipitated by chemical reaction, when people dissect the internal organs of animals, they find that some animals have a film on their intestines and stomach, which can prevent inorganic salts from entering the body. After further research, it is found that the filtration mechanism of this membrane mainly lies in osmotic pressure, that is, fresh water can enter the side with low osmotic pressure through this membrane, but seawater with high salt content cannot. Therefore, people began to study the chemical structure and composition of this film. At present, it has been gradually cracked, and then this membrane is produced for seawater desalination. However, the chemical structure and composition of this membrane have not been completely cracked so far, so it can not achieve the effect of animal membrane. Moreover, the regeneration of this membrane is still in the primary stage, because it cannot be regenerated. After a period of use, it can only be replaced because it is blocked by some inorganic salts and impurities. Moreover, during use, the membrane will gradually block. Therefore, the cost of seawater desalination is still high, but it is believed that human beings can finally break through the difficulties and make seawater desalination technology universal.

Thermal evaporation technology is very simple, that is, heating and evaporating fresh water in seawater and then collecting it. This technology consumes a lot of energy, so the cost is higher than that of membrane technology, and it is generally used in small seawater desalination plants.

China is also studying reverse osmosis technology, and has set up factories in Tianjin, Shandong and other places, and installed some seawater desalination equipment on some islands in the South China Sea to meet the needs of the garrison.

It can be predicted that if the desalination technology is completely broken, China may be able to transfer water from Bohai Bay to Inner Mongolia and Northwest China to improve the ecological environment there. This scheme is much better than the one from the Qinghai-Tibet Plateau, which has little impact on the environment and will not cause disputes in neighboring countries such as India and Bangladesh.

Seawater desalination, also known as seawater desalination, refers to the process of removing excess salts and minerals from water and obtaining fresh water. The process of obtaining fresh water from seawater is called seawater desalination. Seawater desalination is an open source incremental technology to realize the utilization of water resources, which can increase the total amount of fresh water and is not affected by time, space and climate. The water quality is good and the price is gradually reasonable, which can ensure the stable water supply for coastal residents' drinking water and industrial boilers. At present, seawater desalination methods include seawater freezing, electrodialysis, distillation and reverse osmosis. At present, the reverse osmosis method using reverse osmosis membrane has quickly occupied the market with its advantages of simple equipment, easy maintenance and modular equipment, and gradually replaced distillation method as the most widely used method.

The shortage of fresh water resources in the world has become an increasingly worrying problem. It is predicted that there will be a competition for coal in the19th century, oil in the 20th century and water in the 2nd1century.

As an open source incremental technology of water resources, seawater desalination has become an important way to solve the global water crisis. By 2006, seawater desalination technology has been applied in more than 20 countries and regions in the world, and the global daily output of seawater desalination is about 37.75 million tons, of which 80% is used for drinking water, which has solved the water supply problem of 6,543.8+billion people.

"Asking for fresh water from the ocean" has become a new industry. By the end of 2006, China's daily seawater desalination capacity was close to 654.38+500,000 tons, double the previous year. China has made great breakthroughs in key technologies of mainstream seawater desalination, such as reverse osmosis and distillation, and built a 3000 m3/ day low-temperature multi-effect seawater desalination project and a 5000 m3/ day reverse osmosis seawater desalination project with independent intellectual property rights. Seawater DC cooling technology has entered the industrial demonstration stage of10000m3/hour. The cost of seawater desalination in China is gradually decreasing, approaching 5 yuan /m3.

Although China's seawater desalination basically has the conditions for industrial development, there is still a big gap with foreign countries in terms of research level and innovation ability, equipment development and manufacturing ability, system design and integration. The urgent task is to form a complete industrial chain of seawater desalination equipment market in China as soon as possible. Focusing on the key issues that restrict the cost reduction of seawater desalination, we will develop core technologies such as membranes, membrane materials and key equipment, research and develop new technologies, new processes, new equipment and new products for seawater desalination with independent intellectual property rights, improve the localization rate of key materials and key equipment, and enhance the ability to independently build large-scale seawater desalination projects.

There will be nearly 70 billion dollars of business opportunities in the international seawater desalination market in the next 20 years, and China should be able to share a piece of the action. According to the national special plan for seawater utilization, by 20 10, the scale of seawater desalination in China will reach 800,000 to 1000,000 tons/day, and the seawater desalination capacity in China will reach 2.5 million to 3 million tons/day in 2020. In particular, the state actively supports the seawater desalination industry, and the income of seawater desalination projects of enterprises will be exempted from income tax from 2008 1. The development prospect of seawater desalination industry in China is broad.

It is difficult to desalinate seawater, and it is also difficult to scale up, that is, to desalinate seawater on a large scale for human life and production. Seawater is not only salt (sodium chloride), but also other halogen elements and metal ions such as magnesium salt and calcium salt. Fresh water can be obtained by evaporation and condensation on a small scale, and the post-treatment of sediments should be considered on a large scale. Of course, there are efficiency, cost and so on.