CN109920581B - A kind of mixed insulating medium of liquid nitrogen and liquefied carbon tetrafluoride and preparation method thereof - Google Patents

Abstract

A mixed insulating medium of liquid nitrogen and liquefied carbon tetrafluoride and a preparation method thereof are prepared from nitrogen gas with a purity greater than or equal to 99% and carbon tetrafluoride gas with a purity greater than or equal to 99%. The ratio of liquid nitrogen and liquefied carbon tetrafluoride is (2% to 90%): (98% to 10%), which is a molar ratio. When the molar ratio of liquid nitrogen and liquefied carbon tetrafluoride is 45%-90%, the freezing point of the prepared mixed insulating medium can be adjusted to 52K-60K, which meets the cooling requirements of electrical equipment in the temperature area below liquid nitrogen; When the molar ratio of nitrogen and liquefied carbon tetrafluoride is 2%-20%, the normal pressure boiling point of the prepared mixed insulating medium can be adjusted to 89K-120K, which meets the cooling requirements of electrical equipment in the liquefied natural gas temperature zone. The mixed insulating medium of the invention adopts a liquid nitrogen heat exchanger or a refrigerator in a liquid nitrogen temperature zone to liquefy high-purity nitrogen and high-purity CF ₄ ; the high-purity nitrogen and high-purity CF ₄ are determined according to the requirements of the freezing point and bubble point of the mixed insulating medium gas ratio. The mixed insulating medium can be used for electrical equipment in the sub-temperature zone of liquid nitrogen and the temperature zone of liquefied natural gas.

Description

A kind of mixed insulating medium of liquid nitrogen and liquefied carbon tetrafluoride and preparation method thereof

technical field

_The invention relates to a liquid nitrogen and liquefied CF4 mixed insulating medium and a preparation method thereof.

Background technique

Liquid nitrogen is an important low-temperature refrigerant and a liquid insulating material. It is mainly used in electrical equipment such as various superconducting cables, superconducting transformers, superconducting current limiters, and superconducting energy storage devices. By immersing and cooling the superconducting electrical equipment, it provides a low-temperature environment for the superconducting electrical equipment to work normally, and also plays a role in improving the electrical insulation strength, which has a direct impact on the normal operation of the superconducting electrical equipment.

Carbon tetrafluoride (CF ₄ ) is a colorless, odorless, non-flammable, water-insoluble compressible gas at room temperature, which can be used as low-temperature refrigerant and insulating gas. The boiling point of carbon tetrafluoride under normal pressure is about 145K and the melting point is 89.6K. Liquefied carbon tetrafluoride has good electrical insulation properties and can form a eutectic mixture with liquid nitrogen. As shown in Figure 1, its freezing point can reach about 52K , as shown in Figure 2, the highest bubble point can reach about 145K.

Under normal pressure, the boiling point of liquid nitrogen is about 77K, and most high-temperature superconducting materials can work normally at this temperature. Superconducting cables, superconducting current limiters, superconducting transformers, superconducting motors and other electrical equipment are currently directly cooled by liquid nitrogen, while superconducting energy storage (SMES), magnetic resonance (MRI/NMR) imaging systems, etc. The electrical equipment of the magnetic field is mainly cooled by liquid helium (4.2K), liquid neon (20K), and the direct conduction of the refrigerator. Helium and neon are in very low concentrations on Earth and are expensive to liquefy, limiting their large-scale deployment. Compared with the direct immersion of liquid, the electrical equipment directly cooled by the refrigerator has poor thermal stability, and has insulation problems under low vacuum, which also limits its large-scale promotion. Generally speaking, the critical current of high-temperature superconducting materials at the freezing point of liquid nitrogen at 65K is twice that at 77K. If the temperature continues to drop, there is currently no suitable low _- temperature liquid insulating material. Using liquid nitrogen and liquefied CF4 mixed insulating medium, its freezing point It can be further reduced to about 52K. The critical current of high-temperature superconducting materials at 50K self-field is 3-4 times that at 77K. As shown in Figure 3, the current-carrying capacity and magnetic field of superconducting electrical equipment will be greatly improved. , Nitrogen and CF ₄ gas have lower cost, and the insulating ability is much higher than that of helium and neon gas, which is of great significance for the miniaturization and cost reduction of superconducting electrical equipment.

In addition, with the development of superconducting technology, the critical temperature of superconducting materials continues to increase, and it can work in the LNG temperature region (90K-120K) or even higher temperature regions. In this temperature region, a mixed refrigerant throttling refrigerator or an inverse Brayton refrigerator, which is more efficient than a liquid nitrogen refrigerator, can be used for cooling. At higher temperatures, however, corresponding liquid insulating materials are lacking for the insulation and cooling of superconducting electrical equipment. When liquid nitrogen is at 90K, its pressure exceeds 0.3MPa, and at 100K, the pressure exceeds 0.6Mpa, which brings difficulties to the design, manufacture and operation and maintenance of pressure vessels and superconducting electrical equipment. There is an urgent need for a low-temperature insulating cooling medium that can operate in the temperature range of 90K-120K. At present, many Freon refrigerants such as R124, R32, R125, R134a and their mixed refrigerants have poor low-temperature insulation performance, and carbon deposition occurs in strong electric fields, and some low-temperature refrigerants damage the ozone layer and cannot be widely used. high-voltage electrical equipment.

Therefore, it is necessary to develop a new low-temperature liquid insulating material that is green and environmentally friendly, has a simple preparation process, and has a working temperature of 50K to 140K.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a liquid insulation medium for the defects of the existing low-temperature liquid insulating medium that it is difficult to work in the temperature region below liquid nitrogen such as 50K-60K and the temperature region of 90K-120K liquefied natural gas, the electrical insulation performance is poor, and the preparation process is complicated. Nitrogen and liquefied carbon tetrafluoride mixed insulating medium and preparation method thereof.

In order to achieve the above object, the present invention adopts the following technical solutions:

The mixed insulating medium of the present invention is prepared by using nitrogen gas with a purity greater than or equal to 99% and carbon tetrafluoride gas with a purity greater than or equal to 99%, and the ratio of liquid nitrogen and liquefied carbon tetrafluoride is (2% to 90%): (98 %～10%), which is a molar ratio. When the molar ratio of liquid nitrogen and liquefied carbon tetrafluoride is 45%-90%, the freezing point of the prepared mixed insulating medium can be adjusted to 52K-60K, which meets the cooling requirements of electrical equipment in the lower temperature area of liquid nitrogen. When the molar ratio of liquid nitrogen and liquefied carbon tetrafluoride is 2%-20%, the normal pressure boiling point of the prepared mixed insulating medium can be adjusted to 89K-120K, which meets the cooling requirements of electrical equipment in the liquefied natural gas temperature zone. The freezing point and bubble point of the mixed insulating medium are measured by a calorimeter or a phase balance device, and the insulation performance is measured according to the national standard GB/T 507-2002, using a high-voltage test power supply and a standard liquid withstand voltage test electrode. The mixed liquid insulating medium of the invention can not only achieve the dielectric strength equivalent to that of liquid nitrogen, but also can meet the engineering application requirements of 52K-145K in liquefaction temperature.

_The liquid nitrogen and liquefied CF mixed insulating medium of the present invention can be prepared by the following two methods:

method one:

The high-purity nitrogen in the nitrogen cylinder and the high-purity CF ₄ gas in the CF ₄ cylinder are liquefied using a liquid nitrogen heat exchanger. The liquid nitrogen heat exchanger consists of a cryogenic vessel, a liquid nitrogen coil and a mixed insulating medium coil. One end of the liquid nitrogen coil is connected to the liquid nitrogen storage tank, and the other end is empty _; Tube cooling mixed insulating medium coil. Both ends of the liquid nitrogen coil are respectively provided with a liquid inlet valve and an emptying valve, and both ends of the mixed insulating medium coil are respectively provided with an intake valve and a liquid outlet valve. The gas output of nitrogen cylinder and CF ₄ gas cylinder is controlled by their respective pressure reducing valves, and the ratio of high-purity nitrogen gas and high-purity CF ₄ gas is determined by weighing with electronic scale or measuring by gas chromatograph. The liquid outlet pipe of the mixed insulating medium is connected to the mixed insulating medium storage tank. The mixed insulating medium storage tank is a low temperature airtight container, and the top of the low temperature airtight container is equipped with a vacuum valve and a liquid inlet valve. First, the closed container is evacuated to a vacuum state below 0.1Pa through the vacuum unit, and then the mixed insulating medium flows into the insulating medium storage tank through the liquid inlet pipe.

Method Two:

The nitrogen in the nitrogen cylinder and the CF ₄ gas in the CF ₄ cylinder are liquefied by using the refrigerator in the liquid nitrogen temperature zone. The refrigerator in the liquid nitrogen temperature zone is composed of a compressor and a cold head, and nitrogen and CF ₄ gas are cooled by the cold head of the refrigerator. The gas output of nitrogen cylinder and CF ₄ gas cylinder is controlled by their respective pressure reducing valves, and the ratio of high-purity nitrogen gas and high-purity CF ₄ gas is determined by weighing with electronic scale or measuring by gas chromatograph. The gas of the high-purity nitrogen cylinder and the high-purity CF ₄ gas cylinder is connected to the mixed insulating medium storage tank through the gas outlet pipe. The mixed insulating medium storage tank is a low-temperature airtight container, and the top of the low-temperature airtight container is equipped with a vacuum valve and an intake valve; a refrigerator; The cold head is installed on the top of the mixed insulating medium storage tank. First, the closed container is evacuated to a vacuum state below _0.1Pa through the vacuum unit, and then nitrogen gas and CF4 gas flow into the insulating medium storage tank through the intake pipe, and the cold head of the refrigerator Store in storage tanks after condensation.

Compared with other methods, the present invention has the following advantages:

(1) The cost of the mixed insulating medium is low, the freezing point can be as low as about 52K, and its insulating capacity is much higher than that of low-temperature liquids such as liquid helium and liquid neon. Cooling is of great significance for the miniaturization and cost reduction of superconducting electrical equipment.

(2) The boiling point of the mixed insulating medium can reach up to about 145K, which can be easily used for the cooling of electrical equipment in the temperature range of 90K-120K liquefied natural gas, and is of great significance for the development and promotion of electrical equipment in the temperature range of liquefied natural gas.

Description of drawings

Fig. 1 is the solid-liquid phase equilibrium diagram of N ₂ and CF ₄ ;

Fig. 2 is the gas-liquid phase equilibrium diagram of N ₂ and CF ₄ ;

Fig. 3 is the change curve of the critical current of YBCO superconducting tape with temperature and magnetic field;

4 is a schematic diagram of a method for preparing a mixed insulating medium by utilizing a liquid nitrogen heat exchanger in specific embodiment 1 of the present invention;

FIG. 5 is a schematic diagram of a method for preparing a mixed insulating medium using a refrigerator according to Embodiment 2 of the present invention.

Detailed ways

The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

The purity of nitrogen gas and carbon tetrafluoride gas used in the mixed insulating medium of the present invention are both greater than or equal to 99%. Table 1 shows the freezing point, bubble point and DC withstand voltage parameters of the mixed insulating medium with different ratios of liquid nitrogen and liquefied carbon tetrafluoride (CF ₄ ). The mixed liquid insulating medium of the invention can not only achieve the dielectric strength equivalent to that of liquid nitrogen, but also can meet the engineering application requirements of 55K-145K in liquefaction temperature.

Example 1

The mixed insulating medium was prepared with 90% molar ratio of liquid nitrogen and ₁₀ % molar ratio of liquefied CF4. The high-purity nitrogen and high-purity CF4 _were liquefied by the liquid nitrogen heat exchanger method. The freezing point of the prepared mixed insulating medium is 59K, the bubble point at normal pressure is 78K, and the DC withstand voltage is greater than 10kV/mm. The preparation method is as follows:

The high-purity nitrogen in the nitrogen cylinder and the high-purity CF ₄ gas in the CF ₄ cylinder are liquefied using a liquid nitrogen heat exchanger. The liquid nitrogen heat exchanger is composed of a cryogenic vessel, a liquid nitrogen coil and a mixed insulating medium coil. One end of the liquid nitrogen coil is connected to the liquid nitrogen storage tank, and the other end is empty _; The tube cools the mixed insulating medium coil. Both ends of the liquid nitrogen coil are provided with a liquid inlet valve and an emptying valve, and both ends of the mixed insulating medium coil are provided with an intake valve and a liquid outlet valve. The gas output is controlled by the pressure reducing valve of the high-purity nitrogen cylinder and the high-purity CF ₄ gas cylinder. Determine the ratio of high-purity nitrogen gas and high _- purity CF gas by weighing with electronic scale or measuring by gas chromatograph. The liquid outlet pipe of the mixed insulating medium is connected to the mixed insulating medium storage tank, and the mixed insulating medium storage tank is a low temperature airtight container, and the top of the low temperature airtight container is provided with a vacuum valve and a liquid inlet valve. First, the closed container is evacuated to below 0.1Pa, and the mixed insulating medium flows into the insulating medium storage tank through the liquid inlet pipe.

Example 2

The mixed insulating medium was prepared with 80% molar ratio of liquid nitrogen and ₂₀ % molar ratio of liquefied CF4. The prepared mixed insulating medium has a freezing point of 57K, a normal pressure bubble point of 78K, and a DC withstand voltage greater than 10kV/mm. The high-purity nitrogen and high-purity CF ₄ are liquefied by a refrigerator in the liquid nitrogen temperature zone, as follows:

The refrigerator in the liquid nitrogen temperature zone is composed of a compressor and a cold head, and nitrogen and CF ₄ gas are cooled by the cold head of the refrigerator. The pressure reducing valve of the high-purity nitrogen cylinder and the pressure-reducing valve of the high-purity CF ₄ cylinder control the gas output of the cylinder. The ratio of high-purity nitrogen and high-purity CF ₄ gas is determined by weighing with an electronic scale or by gas chromatograph measurement. The gas of the high-purity nitrogen cylinder and the high-purity CF ₄ cylinder is connected to the mixed insulating medium storage tank through the gas outlet pipe. The mixed insulating medium storage tank is a low temperature airtight container with a vacuum valve and an air inlet valve on the top of the low temperature airtight container. The cold head of the refrigerator is installed on the top of the mixed insulating medium storage tank. First, the closed container is evacuated to below _0.1Pa , nitrogen gas and CF4 gas flow into the insulating medium storage tank through the intake pipe, and are stored in the storage tank after being condensed by the cold head of the refrigerator.

Example 3

The mixed insulating medium was prepared with 60% molar ratio of liquid nitrogen and ₄₀ % molar ratio of liquefied CF4. The freezing point of the prepared mixed insulating medium is 54K, the normal pressure bubble point is 79K, and the DC withstand voltage is greater than 10kV/mm. The high-purity nitrogen and high-purity CF ₄ were liquefied by the liquid nitrogen heat exchanger method.

Example 4

_The mixed insulating medium was prepared with 50% molar ratio of liquid nitrogen and 50% molar ratio of liquefied CF4. The freezing point of the prepared mixed insulating medium is 58K, the normal pressure bubble point is 81K, and the DC withstand voltage is greater than 10kV/mm. The high-purity nitrogen and high-purity CF ₄ were liquefied by the refrigerator method in the liquid nitrogen temperature zone.

Example 5

The mixed insulating medium was prepared with ₄₀ % molar ratio of liquid nitrogen and 60% molar ratio of liquefied CF4. The freezing point of the prepared mixed insulating medium is 62K, the normal pressure bubble point is 82K, and the DC withstand voltage is greater than 10kV/mm. The high-purity nitrogen and high-purity CF4 _were liquefied by the liquid nitrogen heat exchanger method.

Example 6

The mixed insulating medium was prepared with ₂₀ % molar ratio of liquid nitrogen and 80% molar ratio of liquefied CF4. The freezing point of the prepared mixed insulating medium is 74K, the normal pressure bubble point is 89K, and the DC withstand voltage is greater than 10kV/mm. The high-purity nitrogen and high-purity CF ₄ were liquefied by the refrigerator method in the liquid nitrogen temperature zone.

Example 7

The mixed insulating medium was prepared with ₁₀ % molar ratio of liquid nitrogen and 90% molar ratio of liquefied CF4. The freezing point of the prepared mixed insulating medium is 81K, the normal pressure bubble point is 97K, and the DC withstand voltage is greater than 10kV/mm. High-purity nitrogen and high-purity CF ₄ are liquefied using a liquid nitrogen heat exchanger.

Example 8

The mixed insulating medium was prepared with ₂ mol% liquid nitrogen and 98 mol% liquefied CF4. The freezing point of the prepared mixed insulating medium is 89K, the normal pressure bubble point is 121K, and the DC withstand voltage is greater than 10kV/mm. The high-purity nitrogen and high-purity CF ₄ were liquefied by the refrigerator method in the liquid nitrogen temperature zone.

Table 1

Claims (3)

1. A mixed insulating medium of liquid nitrogen and liquefied carbon tetrafluoride, characterized in that: the mixed insulating medium is prepared from nitrogen gas with a purity of greater than or equal to 99% and carbon tetrafluoride gas with a purity of greater than or equal to 99% ; The ratio of liquid nitrogen and liquefied carbon tetrafluoride is (2% to 90%): (98% to 10%), which is a molar ratio; the molar ratio of liquid nitrogen to liquefied carbon tetrafluoride is (45%-90 %): (55% to 10%), the freezing point of the prepared mixed insulating medium can be adjusted to 52K-60K, which meets the cooling requirements of electrical equipment in the temperature area below liquid nitrogen; When the molar ratio is (2%-20%): (98%-80%), the atmospheric bubble point of the prepared mixed insulating medium can be adjusted to 89K-121K, which can meet the cooling requirements of electrical equipment in the liquefied natural gas temperature zone; The DC withstand voltage of the prepared mixed insulating medium is greater than 10kV/mm. 2. the compound method of the described mixed insulating medium of claim 1, is characterized in that: utilize liquid nitrogen heat exchanger to liquefy the high-purity nitrogen in nitrogen cylinder and the high _- purity CF gas in CF cylinder _; Liquid nitrogen The heat exchanger is composed of a low temperature container, a liquid nitrogen coil and a mixed insulating medium coil, and the mixed insulating medium coil is cooled by the liquid nitrogen coil; The two ends of the mixed insulating medium coil are respectively provided with an inlet valve and a liquid outlet valve; the gas output of the nitrogen cylinder and CF ₄ gas cylinder is controlled by their respective pressure reducing valves, and the ratio of high-purity nitrogen and high-purity CF ₄ gas is controlled by electronic Weighing or gas chromatograph to determine; the liquid outlet pipe of the mixed insulating medium is connected to the mixed insulating medium storage tank, the mixed insulating medium storage tank is a low-temperature airtight container, and the top of the mixed insulating medium storage tank has a vacuum valve and a liquid inlet valve; First, the mixed insulating medium storage tank is evacuated to a vacuum state below 0.1Pa by the vacuum unit, and the mixed insulating medium flows into the insulating medium storage tank through the liquid inlet pipe. 3. the compound method of the described mixed insulating medium of claim 1, is characterized in that: utilize the refrigerator of liquid nitrogen temperature zone to liquefy the high-purity nitrogen in nitrogen cylinder and the high _- purity CF gas in CF cylinder _; The refrigerator in the liquid nitrogen temperature zone is composed of a compressor and a cold head. The nitrogen and CF ₄ gas are cooled by the cold head of the refrigerator _; The ratio of nitrogen and high-purity CF ₄ gas is determined by electronic scale weighing or gas chromatograph measurement; the gas of nitrogen cylinder and CF ₄ gas cylinder is connected to the mixed insulating medium storage tank through the gas outlet pipe, and the mixed insulating medium storage tank is sealed at low temperature The top of the container is equipped with a vacuum valve and an intake valve; the cold head of the refrigerator is installed on the top of the mixed insulating medium storage tank; first, the mixed insulating medium storage tank is evacuated to a vacuum state below 0.1Pa by a vacuum unit, nitrogen and _The CF4 gas flows into the insulating medium storage tank through the intake pipe, and is condensed by the cold head of the refrigerator and stored in the storage tank.

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