Liquid nitrogen is a liquid at low temperatures - 196 degrees Celsius. It is derived from nitrogen, which occupies more than 70% of the air. Inert, colorless, non-toxic, volatile, very wide range of use. Liquid nitrogen is volatile at low temperature, so it is generally stored in a special container - liquid nitrogen tank.

According to the use of liquid nitrogen purity, it is usually divided into food grade liquid nitrogen and industrial grade liquid nitrogen. The purity of food-grade liquid nitrogen is relatively high, because considering the safety of human consumption, the purity reaches 5.9, or 99.999%. The purity of industrial grade liquid nitrogen is not as high as that of food grade liquid nitrogen. Different industries have different requirements for the purity of liquid nitrogen.

Food-grade liquid nitrogen applications such as liquid nitrogen ice cream, liquid nitrogen biscuits, medical aspects of liquid nitrogen freezing, anesthesia also need high purity of liquid nitrogen. Other industries such as chemical industry, electronics, metallurgy have different requirements for the purity of liquid nitrogen.

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I. Application in Animal Husbandry

1. Widely used in livestock frozen mix improvement technology

Liquid nitrogen is widely used as a cold source for bovine semen freezing and can be removed whenever necessary.

2. In the embryo transfer of livestock and many kinds of animals, most of the embryos are prepared and preserved by embryo freezer, which belongs to intelligent freezer. The instrument adopts computer control technology and special software, which can control the amount of liquid nitrogen accurately, so as to ensure that frozen biological products are cooled and frozen at an appropriate freezing rate.

3. Liquid nitrogen cryopreservation microbial technology

4. Conservation of Agrobiological Genes

5. Preservation of liquid nitrogen vaccine

6. It is also common to use liquid nitrogen to treat closure of papillary orifice of dairy cows and skin tumors of livestock.

II. Application in Medical Career

1. Hypothermia Medicine

The rapid development of clinical hypothermia medicine in China has promoted the development of transplantation medicine, especially the cryopreservation and transplantation of bone marrow, hematopoietic stem cells, skin, cornea, endocrine glands, blood vessels and valves. Successful hematopoietic stem cell transplantation depends on the preservation of hematopoietic stem cell viability. During the cooling and freezing process of biological samples, a certain amount of heat will be released during the phase transition from liquid to solid, which will make the temperature rise again. The freezing process without controlling the cooling rate will lead to the death of tissue cells. Accurate determination of phase transition point of biological samples and compilation of cooling program with computer can increase liquid nitrogen input during phase transition, overcome the temperature rise of phase transition samples, and make cells pass the phase transition safely and quickly. This is the key link to improve the survival rate of frozen samples.

2. Clinical Medicine

Liquid nitrogen is the most widely used cryosurgical agent in cryosurgery. It is the best refrigerant found so far. It can be injected into a cryogenic medical device. Like a scalpel, it can perform any operation. Cryotherapy is a method of treating lesions that are destroyed by low temperature. Because of the rapid change of temperature, crystal formation inside and outside tissues, dehydration and shrinkage of cells, electrolyte changes, freezing can also slow down the local blood flow, blood stasis or embolization of microvasculature can cause cell death due to hypoxia.

Liquid nitrogen freezing combined with traditional Chinese medicine in the treatment of condyloma acuminatum has no toxic side effects, less pain, high cure rate, low recurrence rate, no scar after operation, fewer complications, short treatment time and economy. It is one of the ideal methods for the treatment of condyloma acuminatum.

Liquid nitrogen cryotherapy is used to treat chronic pharyngitis in the treatment of nodular itching, mucocele, multiple verruca vulgaris and neurodermatitis. The liquid nitrogen is directly sprayed on the surface of the lesion. The cooling rate is fast, deep and wide, and the middle zone can be eliminated. Satisfactory short-term results have been achieved. Each freeze-thaw cycle is controlled in about 1 minute and lasts 4-5 consecutive freeze-thaw cycles. Only once a week.

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Application in Food Industry

1. Application of liquid nitrogen in food quick-freezing

Among many preservation methods, freezing preservation method is the most widely used and the effect is very significant. As one of the methods of freezing and preservation, liquid nitrogen quick-freezing has been used by food processing enterprises for a long time. Because it can realize super-freezing at low temperature and deep cooling, it is also conducive to the partial vitrification of frozen food, so that the food can be restored to its original fresh state and original nutrients after thawing, and greatly improve the quality of frozen food, so it shows in the quick-freezing industry. Unique vitality.

2. Application of liquid nitrogen in beverage processing

At present, many beverage manufacturers have replaced the traditional C02 with a mixture of nitrogen or nitrogen and C02 to pack the beverage by air. High-carbonated drinks filled with nitrogen cause fewer problems than those filled with carbon dioxide only; nitrogen is also ideal for filling non-foaming drinks, such as wine and juice. The advantage of adding liquid nitrogen to non-inflatable beverage cans is that a small amount of liquid nitrogen injected can eliminate oxygen in the top space of each can, making the gas in the upper space of the tank inert, thus prolonging the storage life of perishable products.

3. Application of Liquid Nitrogen in Storage and Fresh-keeping of Fruits and Vegetables

The application of liquid nitrogen in the storage and fresh-keeping of fruits and vegetables has the advantage of adjusting atmosphere, which can adjust the contradiction between supply and demand of agricultural by-products in peak and off-season, and reduce the loss of storage. The function of air conditioning is to increase the concentration of nitrogen, control the proportion of nitrogen, oxygen and CO2, and keep them in a stable state, reduce the respiratory intensity of fruits and vegetables, delay the Post-ripening process, so as to keep fruits and vegetables fresh and original nutritional value during picking, and prolong the fresh-keeping period of fruits and vegetables.

4. Liquid nitrogen in meat

The quality of raw meat can be effectively improved by adding liquid nitrogen in the process of raw meat grinding, chopping and mixing. For example, in the processing of salami sausage, the use of liquid nitrogen can improve the water retention of meat, prevent fat oxidation, improve the slicing and appearance quality; when used in the processing of meat snacks, preserved meat and other processed meat, it can not only accelerate protein dissolution and enhance water retention when minced meat is mixed, but also is particularly effective in maintaining the unique shape of the product. In addition, the fast cooling of raw meat by liquid nitrogen can not only maintain the hot meat characteristics for a long time, but also ensure the health and safety of meat products. In processing technology, there is no need to worry about the effect of temperature rise on meat quality, and processing can not be affected by raw material temperature, processing time and seasonal factors. At the same time, it can also make the processing process in low oxygen partial pressure and prolong the shelf life of products in a certain range. Life.

5. Application of Liquid Nitrogen in Food Crushing at Low Temperature

Low-temperature crushing is the process of crushing the material cooled to the brittle point temperature into powder under the action of external force. Low temperature comminution of food is a new food processing technology developed in recent years. This technology is especially suitable for processing foods with more aromatic components, higher fat content, more sugar content and more gelatinous substances. Low temperature crushing with liquid nitrogen can crush all the raw materials such as bone, skin, meat and shell at one time, so that the finished granules are fine and keep their effective nutrition. For example, the seaweed, chitin, vegetables and condiments frozen by liquid nitrogen in Japan are put into the crusher to crush, which can make the fine particle size of the finished product up to 100 microns or less, and basically maintain the original nutritional value. In addition, low-temperature comminution with liquid nitrogen can also comminute materials that are difficult to comminute at room temperature, thermal sensitivity, heat-sensitive, easily deteriorated and decomposed. In addition, liquid nitrogen can crush food raw materials that are difficult to crush at room temperature, such as fat-rich meat and moisture-rich vegetables, and can also produce new processed foods that have not been produced so far.

6. Application of Liquid Nitrogen in Food Packaging

A company in London, UK, has developed a simple and practical method of food preservation packaging, in which a few drops of liquid nitrogen are added to the packaging bag when the food is packaged. When liquid nitrogen evaporates into gas, its volume expands rapidly, replacing most of the original gas in the packaging bag, reducing the deterioration of food caused by oxidation, thus greatly prolonging the shelf life of food.

7. Application of Liquid Nitrogen in Food Refrigeration Transportation

Refrigerated transportation is an important part of the food industry. Developing liquid nitrogen refrigeration technology, developing liquid nitrogen refrigerated trains, refrigerated vehicles and refrigerated containers are the common development trends in the world today. The application practice of developed countries for many years shows that liquid nitrogen refrigeration system is the only refrigeration technology that can compete with mechanical refrigeration system commercially, and it is also the development direction of food refrigeration and transportation.

8. Other Applications of Liquid Nitrogen in Food Industry

Owing to the refrigeration effect of liquid nitrogen, egg juice, liquid condiment and soy sauce can be processed into granular frozen food which can flow freely and pour out. These food can be used at any time and can be easily prepared. When grinding spices and absorbent food additives such as sugar substitutes and lecithin, liquid nitrogen is injected into the grinder to protect valuable ingredients and increase grinding output. Zhou Shunhua et al. have proved that using liquid nitrogen quenching and high temperature thawing to break pollen wall has the characteristics of good effect, high breaking rate, fast speed, stable physiological activity of pollen and no pollution.

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IV. Application in Electronic Industry

1. Superconducting Technology

   
   

Superconductor has unique characteristics, which make it possible to be widely used in various fields. Using liquid nitrogen instead of liquid helium as superconducting refrigerant to obtain superconductors makes superconducting technology go to large-scale development and application, which is considered as one of the greatest discoveries in Science in the 20th century.

2. Manufacturing and Testing of Electronic Components

Liquid nitrogen is a faster and more effective method for shielding and testing electronic components and circuit boards.

3. Low Temperature Ball Milling Technology

The cryogenic planetary ball mill is a planetary ball mill equipped with thermal shield. These cold gases absorb the heat generated by the high-speed rotating ball mill in time, so that the ball mill with materials and grinding balls is always in a certain cryogenic environment. The necessary equipment for mixing, fine grinding, new product development and small batch production of high-tech materials in low temperature environment. The product is small in size, full in function, high in efficiency and low in noise. It is widely used in medicine, chemical industry, environmental protection, light industry, building materials, metallurgy, ceramics, minerals and other departments.

4. Green Cutting Technology

Low-temperature cutting is a cutting area which uses cryogenic fluids such as liquid nitrogen, liquid carbon dioxide and cold air to spray into the cutting system, resulting in local cryogenic or ultra-cryogenic state of the cutting area. The low-temperature brittleness of the workpiece under low-temperature conditions is utilized to improve the workpiece's machinability, tool life and surface quality. There are two main applications of liquid nitrogen cooling cutting, one is to use bottled pressure to inject liquid nitrogen directly into the cutting area like cutting fluid; the other is to use liquid nitrogen heating evaporation cycle to indirectly cool the tool or workpiece. At present, low-temperature cutting is mainly used in the processing of difficult-to-machine materials such as titanium alloy, high manganese steel and hardened steel.

V. Other Areas

The special fuel station of Jiuquan Satellite Launch Center produces liquid nitrogen, which is the propellant of rocket fuel. A large amount of liquid nitrogen pushes rocket fuel into the combustion chamber with high pressure.