Rapid Recovery Technology of SF6/N2 Mixed Insulation Gas in Electrical Equipment

SF6 mixed gas is widely used as an insulation and arc extinguishing medium in electrical equipment in cold regions of northern China. Due to its low liquefaction temperature, existing gas recovery technologies are unable to achieve efficient and rapid recovery of SF6/N2 mixed insulation gases. In response to this challenge, a two-stage membrane separation and recovery technology has been proposed, which can efficiently and quickly separate SF6 and N2 from the mixed gas. The volume fraction (V/V) of obtained product SF6 gas reaches 90%, and the use of compression and refrigeration units can achieve rapid filling of 50 m3/h mixed gas. At the same time, the enrichment and recovery of trace SF6 gas in the exhaust gas are achieved. After enrichment, the volume fraction (V/V) of SF6 gas reaches 92%, and the volume fraction (V/V) of SF6 gas in the emptied mixed gas is 600 μL/L, far below the national standard value, solving the problems in actual production.


Introduction
SF6 gas is widely used in high-voltage electrical equipment due to its excellent insulation and arc extinguishing characteristics [1] .However, low temperatures in northeast regions of China can cause liquefaction of SF6 gas in equipment, and sensitivity to uneven electric fields, leading to a decrease in equipment insulation performance and bringing safety hazards [2,3] .SF6 gas is a greenhouse gas, whose greenhouse effect is 23, 900 times that of CO2, while SF6/N2 mixed insulation gas has good insulation, arc extinguishing, and distortable electric field performance [4] .It can also effectively solve the problem of equipment insulation failure caused by the liquefaction of pure SF6 gas in high-altitude and cold regions [5] .At the same time, it has significantly reduced the use of greenhouse gas SF6.Currently, it has been widely promoted and applied in some regions of China.
During the maintenance process, power grid enterprises utilize existing SF6 gas recovery technology to recover SF6/N2 mixed insulation gas.They found that the current recovery technology has low efficiency and slow speed, and cannot meet the existing production needs [6] .To solve the above problems, there is an urgent need for a new gas recovery technology SF6/N2 difficulty in recycling mixed insulation gases.Membrane separation technology has advantages such as green environmental protection and efficient separation, providing a solution for this project.

Existing technology for recycling SF6/N2 mixed insulation gas test
We recycle SF6/N2 mixed insulation gas (SF6:N2=1:1) with recycling simulation equipment using existing SF6 gas recovery technology, and the equipment inflation pressure is 0.6 MPa with a volume of 1 m 3 .Then we fill the recovered mixed gas into a 40 L steel cylinder, as shown in Figure 1.

Figure 1.
The existing recovery technology of SF6/N2 insulating mixed gas.In Figure 1, after 15 minutes of recycling, only 10% mixture of the simulated equipment is recovered, and the pressure inside the cylinder has reached 4.8 MPa.Continue compression recycling is difficult to sustain.Therefore, it can be seen that due to the difficulty of effectively compressing liquefied gas mixtures using existing technologies, the collection speed with the return of existing SF6 gas recovery technology is slow and time-consuming, and unable to meet SF6/N2 mixed gas recovery job requirements.

A new recycling technology of SF6/N2 mixed gas recovery
A new recycling technology of SF6/N2 mixed gas recovery that utilizes two stages of membrane separation technology, compression refrigeration technology, and other technologies has achieved SF6/N2 mixed isolation.The new recycling technology of SF6/N2 mixed gas recovery that is efficient and rapid recovery meets practical needs.
Membrane separation technology has the characteristics of efficient separation and environmental friendliness, which is suitable for the separation and enrichment of low-concentration gas components.Polycarbonate hollow fiber membrane is used to separate the SF6/N2 insulation mixture.SF6/N2 insulation mixture gas and SF6 gas decomposition products have different diffusion rates on both sides of the membrane under pressure, meaning that the separation coefficient between SF6 gas and other component gases is greater than 3.The separation coefficient between SF6 gas and other component gases is shown in Table 1.In Table 1, the separation factors of N2, CO, CO2, H2S, SO2, and H2O are all greater than 3. Therefore, polycarbonate hollow fiber membranes can be used to separate the SF6/N2 insulation mixture.
Table 1.Separation factors of SF6 gas and related gas components.

Design of SF6/N2 mixed insulating gas rapid recovery system
The system consists of four modules: a mixed gas pretreatment module, a mixed insulation gas rapid separation module, a filling module, and a tail gas deep treatment module.Figure 2 is the workflow of each module.

Pre-process module
The mixed insulation gas entering from the air inlet is filtered and adsorbed by the pre-processor to remove impurities such as decomposition products and moisture from the gas.We connect to a large flow compressor unit to provide the pressure required for membrane separation, which should not exceed 1.2 MPa, and remove dust (particle size>0.01μm) from the mixed gas through a precision filter to protect the quality of the membrane.Finally, we enter the heating module and control the gas temperature based on the operating temperature of the membrane separation device.

Membrane separation module
After the mixed gas enters the STAGE 1 membrane for filtration, it is divided into permeable gas 1 and outlet gas 1.Among them, permeable gas 1 contains 0.95% SF6 (V/V) and 99.05% N2, which does not meet the emission standards and needs to be cached and entered into the exhaust gas advanced treatment unit for treatment; outlet gas 1 contains 71.71% SF6(V/V), which cannot achieve fast and efficient filling.Outlet gas 1 enters the STAGE 2 membrane, which is divided into permeate gas 2 and outlet gas 2. Permeate gas 2 containing 4.85% SF6 (V/V) enters the pre-treatment module again for cyclic separation; outlet gas 2 containing 90.91% SF6 (V/V) enters the compression cooling filling module.The separation test flow of mixed insulating gas (SF6/N2) is shown in Figure 2.

Filling module
After outlet gas 2 enters the filling module, it is stored after compression and cooling.

Exhaust gas advanced treatment module
The exhaust deep treatment module mainly includes membrane components 3 and membrane components 4. The exhaust gas treatment module mainly deals with air permeability.It contains 0.95% SF6 (V/V) and 99.05% N2(V/V) and enters membrane components 3 after the temperature and pressure control module through a buffer tank (pressure of 0.5 MPa, temperature of 45℃).
It is divided into permeable gas 3 and outlet gas 3, with permeable gas 3 containing 600 μL/L SF6, which is lower than the national standard requirement of 1000 μL/L and can be directly discharged.Outlet gas 3 contains 9.31% SF6 (V/V), which is divided into permeate gas 4 and outlet gas 4 after entering the membrane components 4.Among them, permeate gas 4 contains 2.21% SF6 (V/V).It can enter the STAGE 1 membrane through a pipeline and undergo cyclic separation again.The SF6 gas content in outlet gas 4 is 92.77% (V/V), which can be stored through filling.

Recycling performance test
Firstly, we prepare a mixed insulation gas of 50% SF6+50% N2, store it at 2 m³ in the storage tank, and stabilize the pressure and flow rate passing through the 2 m³ test tower.After passing through the test tower and pre-treatment module in sequence, it enters the recovery unit and tail gas deep treatment unit.According to the separation characteristics of the membrane, the gas pressure and temperature are controlled to 1.2 MPa and 45℃.The gas concentration and flow rate at the inlet (feed gas), STAGE 1 membrane population (population gas 1), permeate gas 1, outlet gas 1, STAGE 2 membrane population (intake gas 2), permeate gas 2, and outlet gas 2 positions are measured, respectively.The raw gas and inlet gas 1 have not undergone any treatment, so the measurement results of concentration and flow remain consistent.The SF6/N2 in the gas is 1:1, and the flow rate is 50 m 3 /h.After the mixed gas is separated by the STAGE 1 membrane, the volume fraction of SF6/N2 in outlet gas 1 is 71.71:28.29 and the flow rate is 34.65 m 3 /h.The SF6/N2 in permeate gas 1 is 0.95:99.05and the flow rate is 15.35 m 3 /h.Due to the permeate gas not meeting the emission standard, it enters the exhaust gas deep treatment module for further treatment.After outlet gas 1 enters the STAGE 2 membrane for separation, the volume fraction of SF6 further increases.The volume fraction of SF6 in outlet gas 2 is 90.91%, with a flow rate of 26.93 m 3 /h, which can meet the requirements for gas compression, liquefaction, and filling.The SF6/N2 content in permeate gas 2 is 4.85:95.15,with a flow rate of 7.71 m 3 /h, which does not meet the emission standard and enters the re-filtration purification process again.The gas storage tank and test tower are shown in Figure 3. Membrane separation device is shown in Figure 4. Gas concentration in different outer system locations is shown in Figure 5.

Advanced treatment of exhaust gas
Permeable gas 1 enters the tail gas treatment process.We control the pressure to 0.5 MPa and the temperature to 45℃.The gas concentration and flow rate at the raw gas inlet (feed gas 2), STAGE 3 membrane inlet (inlet gas 3), permeable gas 3, outlet gas 3, permeate gas 4 inlet (outlet gas 3), permeable gas 4, and outlet gas 4 positions are measured, respectively.Raw gas 2 and inlet gas 3 have not undergone any treatment, so the concentration and flow measurement results remain consistent.The SF6/N2 in the gas is 0.95:99.05,and the flow rate is 15.4 m 3 /h.After the mixture is separated by the STAGE 3 membrane, the volume fraction of SF6/N2 in outlet gas 3 is 9.31:90.69,the flow rate is 1.4 m 3 /h, and it enters STAGE 4 for further separation and treatment.The volume fraction of SF6/N2 in permeate gas 3 is 0.05:99.95,with a flow rate of 14 m 3 /h, which meets the emission standard and is directly discharged.After outlet gas 3 enters the STAGE 4 membrane for separation, the concentration of SF6 further increases, the concentration of SF6 in outlet gas 4 is 92.77%, and the flow rate is 0.11 m 3 /h.It can meet the requirements of gas compression liquefaction filling.The SF6/N2 in permeate gas 4 is 2.21:97.79,with a flow rate of 1.28 m 3 /h, which does not meet the emission standard and enters the re-filtration purification process again.Depth treatment for exhaust gas is shown in Figure 6.

Conclusion
The recovery technology for separating mixed gases is achieved by using a hollow fiber membrane made of polycarbonate.During use, the membrane has a separation factor between the main gas and the gas that needs to be separated α greater than 3.A membrane separation device was designed in the experiment, with fast separation speed and a processing capacity of 50 m 3 /h.The mixed gas recovery efficiency of SF6 can reach 26.93 m³/h.It can achieve rapid recovery and separation of mixed insulation gases in electrical equipment by using SF6/N2 mixed gas as an insulation medium, effectively reducing the greenhouse effect impact on the environment.
The SF6/N2 mixed insulation gas recovery system can utilize two-stage membrane separation technology, compression refrigeration technology, and other technologies to achieve efficient and rapid recovery of SF6/N2 mixed insulation gas, solving the problem of SF6/N2 mixed insulation gas recovery and meeting practical needs.

Figure 3 .
Figure 3. Gas storage tank and test tower.Figure 4. Membrane separation device.

Figure 4 .
Figure 3. Gas storage tank and test tower.Figure 4. Membrane separation device.

Figure 5 .
Figure 5. Gas concentration in different outlet system locations.

Figure 6 .
Figure 6.Depth treatment for exhaust gas.