What are the advantages of desalination? What are the methods of desalination?

Desalination can alleviate water scarcity incrementally; The seawater reserves are huge, the cost of obtaining them is zero, and there is no need to pay any party; Seawater resources know no borders; Seawater desalination production has strong stability, and is little affected by raw materials and natural climatic conditions; The water quality of desalinated water is pure. From an international point of view, the application of large-scale seawater desalination has been successfully practiced. Currently, Saudi Arabia, Israel.

and other countries 70 of the freshwater resources.

From seawater desalination, the United States, Japan, Spain and other countries are also competing to develop seawater desalination industry in order to protect their own freshwater resources. In 2008, the total investment in global desalination projects reached 24.8 billion US dollars, and the annual growth rate is 20 30 US dollars, and it is expected to reach 56.4 billion US dollars in 2015. The total output of seawater desalination in the world reached 63.48 million tons per day, and seawater cooling water.

The annual consumption is more than 700 billion tons, and the annual seawater salt production is 60 million tons, magnesium 2.6 million tons, and bromine.

500,000 tons. From a domestic point of view, in recent years, seawater desalination and comprehensive utilization have made great progress.

1. For coastal areas and islands where freshwater resources are scarce and the urban water pipe network cannot be extended, the use of seawater desalination equipment can solve the problem of water for residents' life and production.

2. In terms of land occupation, traditional water supply facilities occupy a large amount of farmland, long-distance water pipeline networks occupy a large amount of land, and seawater desalination equipment generally does not need to build long-distance water pipeline networks, and can be directly connected to urban pipe networks; From the perspective of capital occupation, the construction of reservoirs relies on financial investment, and requires a large amount of investment in maintenance every year, and seawater desalination equipment can use project financing and other methods to invest only part of the financial resources.

3. Factories in coastal areas can use seawater desalination equipment to use desalination seawater as daily production and domestic water, which not only saves water resources, but also saves water costs.

4. Although the South-to-North Water Diversion Project can help water-scarce areas, far water cannot quench the thirst of the near-term, and the water resources remain unchanged in terms of quantity. Seawater desalination equipment can solve the water problem of cities with poor freshwater resources.

(1) Freezing method.

Freezing is the freezing of seawater, in which the salt is separated as the liquid seawater becomes solid ice.

2) Reverse osmosis.

Commonly known as ultrafiltration, this method uses a semi-permeable membrane that only allows solvents to pass through and does not allow solutes to pass through to separate seawater from fresh water. Under normal circumstances, fresh water diffuses to the seawater side through a semi-permeable membrane, so that the liquid level on the seawater side gradually rises until a certain height does not stop, this process is called osmosis. If an external pressure greater than the osmotic pressure of the seawater is applied to one side of the seawater, then the pure water in the seawater will reverse osmosis into the fresh water.

The biggest advantage of the reverse osmosis method is energy saving.

3) Solar Energy Law.

Solar distillation is the use of a simple solar still. The still consists of a tank with a black porous felt core floating cavity covered by a transparent, closed glass covering the top. Sunlight is projected through the transparent overlay onto the bottom of the black, adiabatic trough and converted into heat.

4) Low temperature distillation.

Low-temperature multi-effect distillation desalination technology.

Low-temperature multi-effect seawater desalination technology refers to the distillation desalination technology with the highest evaporation temperature of brine lower than 70, which is characterized by a series of horizontal tube spray falling film evaporators connected in series, with a certain amount of steam input to the first effect, and the evaporation temperature of the latter effect is lower than the previous effect, and then through multiple evaporation and condensation, so as to obtain the desalination process of distilled water with multiple times the amount of steam.

5) Multi-stage flash evaporation.

Flash evaporation refers to the phenomenon of rapid evaporation of part of seawater at a certain temperature under the condition of sudden decrease in pressure. Multi-stage flash seawater desalination is to evaporate heated seawater in multiple flash chambers with gradually decreasing pressure, and condense steam to obtain fresh water.

Among the more than 20 desalination technologies that have been developed, distillation, electrodialysis, and reverse osmosis have reached industrial-scale production applications.

Electrodialysis desalination is achieved using a specially manufactured thin film. Under the action of electricity, the positive ions of salts in seawater run through the positive membrane to the direction of the cathode, and cannot pass through the negative membrane and stay; Negative ions run through the anion to the anode direction, and cannot pass through the positive membrane and stay. In this way, the seawater in the pipeline where the salt ions are exchanged becomes fresh water, and the seawater in the pipeline where the salt ions are left becomes the concentrated brine.

Reverse osmosis desalination is even more subtle. The membrane it uses is called a "semi-permeable membrane". The performance of semi-permeable membranes is to allow only fresh water to pass through, not salts.

If no pressure is applied, this membrane is used to separate the brackish water from the fresh water, and the fresh water will automatically penetrate the brackish water. We apply pressure to the seawater through a high-pressure pump, and the fresh water in the seawater passes through the membrane to the fresh water, so it is called reverse osmosis, or reverse osmosis.

The principle of distillation is very simple, it is the principle that we prepare distilled water in the laboratory. The seawater is boiled, the fresh water evaporates into steam, the salt remains at the bottom of the pot, and the steam condenses into distilled water, which is fresh water. This ancient desalination method consumes a lot of energy and produces a lot of pot scale, making it difficult to produce fresh water in large quantities.

Modern multi-stage flash distillation desalination has rejuvenated the ancient distillation method. Water is heated to 100 degrees Celsius at normal air pressure before boiling to become steam. If properly heated seawater enters a vacuum or a near-vacuum distillation chamber, it will rapidly evaporate into steam in an instant.

Using this principle, a multi-stage flash distillation desalination device is made. This kind of desalination device can be built relatively large, and the vacuum evaporation chamber can be built more, connected to become a large-scale desalination plant. This kind of desalination plant can be built in conjunction with a thermal power plant to use the waste heat of the thermal power plant to heat seawater.

Combined hydropower can greatly reduce production costs. Most of the current large-scale desalination plants use this method. If the solar evaporation desalination method can be put into practice, the ancient distillation desalination technology will reach a new level of energy saving.

At present, seawater desalination has basically formed a new industrial sector. In 1975, the world's daily desalination production reached 2 million tons. Since then, it has grown at a rate of doubling every two years.

In 1975, Hong Kong built a desalination plant with a capacity of 180,000 tons per day, which was the world's most advanced desalination plant at that time. In 1983, a 300,000-ton-per-day desalination plant was built at the port of Jeddah, Saudi Arabia. Kuwait's seawater desalination capacity is more than 1 million tons per day.

The oil-rich countries in the Middle East have basically relied entirely on seawater desalination for their daily water.

Hope it helps.

In fact, the most advanced ones at present are the membrane method and the thermal method, and the other prospects do not seem to be great at present. The advantages of the thermal method are: first, it can use low-grade waste heat (it can be coupled with solar or nuclear waste heat); Second, it can be applied on a large scale, such as urban water supply; Third, the output water quality is high and stable.

Disadvantages: The construction location depends on the heat source, and the initial investment is large. Advantages of the membrane method:

There are many restrictions and low costs. Disadvantages: It needs to be replaced, and the water quality will decrease with the use time of the membrane.

However, with the improvement of the technical level of polymer technology materials, this shortcoming is becoming less and less obvious.