Oil and gas storage tanks will continuously accumulate heat at high temperatures, resulting in rapid temperature rise on the surface and inside. In order to solve the problem of internal pressure increase caused by high temperature, it is necessary to open the vent valve frequently, which will aggravate product loss and cause air pollution. When the internal pressure exceeds the critical value, it may even cause an explosion. Generally, the storage temperature of light boiling point oil in tanks should not be higher than 45℃, and the temperature difference between inside and outside liquefied natural gas (LNG) tanks can even reach 200℃. In addition, there is a large temperature difference between day and night in summer, and the “breathing phenomenon” of oil tanks will lead to an increase in the concentration of combustible oil and gas in the vicinity of the oil tank farm, which wastes oil and gas resources and also brings potential safety hazards.
In recent years, scholars at home and abroad have proposed spraying cold water to reduce the temperature of tanks. The disadvantage of this method is that rust and dirt will adhere to the outer wall of the oil tank. On the one hand, the tank wall is moldy and rusted, and the service life of the equipment is shortened; On the other hand, dirt darkens the color of the tank wall, which intensifies the absorption of sunlight by the tank wall and causes local temperature rise. These practices have not fundamentally solved the problems of oil and gas loss and potential safety hazards. In addition, spraying water will also cause a huge waste of water and electricity resources. Among many related technologies, coating cooling has the advantages of low cost, remarkable effect and convenient transformation, and has a good application prospect in the field of building and industrial energy saving.
Radiation refrigeration coating is a new type of coating based on passive radiation refrigeration technology. It has high infrared emission capability in the atmospheric transmission window band (8-13 μm), and can continuously and efficiently transfer heat to cold sources in outer space. At the same time, it has a high reflection ability to solar radiation energy, which can minimize the heat absorption of solar radiation and realize 24-hour zero-energy radiation refrigeration. The demands of temperature reduction and emission reduction in the petrochemical industry are highly consistent with the metamaterial technology of radiation refrigeration. On the one hand, passive radiation refrigeration technology can provide significant temperature “peak elimination” during the day, effectively reduce the risk that the temperature of the working medium in the tank exceeds the standard, and improve the safety and reliability of equipment operation; On the other hand, it can also effectively inhibit the “respiration” of oil and gas caused by the temperature difference between day and night, reduce the emptying of materials, and relieve the environmental protection discharge pressure of enterprises. This technology has great economic and social significance.
1. Surface temperature influences mechanism.
The temperature of the outer wall of the tank has a great influence on the working medium state in the tank. Chen Qisheng clarified through modeling that the heat from the outside environment permeates into the tank wall and causes the evaporation loss of oil and gas in the tank. At the same time, the heat infiltration will cause the internal pressure of the tank to further increase, thus posing a potential safety hazard. The thermal resistance of the tank wall can be calculated by the formula (1):
Ｒ = 1 / ( kmS / Δh + ks Ss / Δh)
km, ks: the flatness of insulation layer and steel structure of storage tank.
S, Ss: the area of the inner surface of the storage tank and the steel structure.
Δh: the total thickness of the insulation layer.
According to the formula, when the tank structure is determined, the heat flow into the tank is directly proportional to the temperature difference between inside and outside the tank (δ t). Therefore, finding a convenient and efficient method to reduce the temperature of the outer wall of the storage tank is of great significance for reducing the temperature of the working medium in the tank and reducing the evaporation loss of oil and gas. Epoxy zinc-rich paint is usually sprayed on the surface of oil and gas storage tanks, which plays a role in preventing corrosion and increasing tank wall reflection. Compared with epoxy zinc-rich paint, radiation refrigeration paint has higher solar reflectivity. In the visible light band, where the solar energy is mainly concentrated, the reflectivity of the radiation refrigeration coating is close to 95%. Therefore, compared with the common epoxy zinc-rich paint, the radiation refrigeration coating will greatly reduce the solar heat gain and change the heat flow direction of the tank.
Theoretically, if the solar radiation intensity is 900-1 000 W/ m2 at noon, only when the solar reflectivity of the coating is greater than 88% can refrigeration be realized. According to the optical data, the solar reflectivity of the radiation refrigeration coating is as high as 93%, and the emissivity of the infrared window is as high as 96%, which can realize the refrigeration function that other coatings can’t achieve without energy consumption.
The cooling effect of radiation cooling paint is not possessed by ordinary heat-insulating white paint. For the surface of ordinary heat reflective white paint, when the solar radiation acts on the surface of the white paint of the tank, most of the energy will be absorbed, which will cause the tank to heat up and become a “heat source”. Part of the absorbed solar radiation energy is lost with air convection, which heats the surrounding air and affects the tank again. The other part is gradually conducted downward into the tank, resulting in an increase in the temperature of oil and gas in the tank. When the surface of the storage tank is sprayed with radiation refrigeration coating, the solar reflectivity is as high as 93%, which makes the surface of the storage tank basically not absorb solar radiation. At the same time, the emissivity of the infrared window (close to blackbody) of radiation refrigeration coating is as high as 96%, which makes the heat transfer to outer space continuously and reduces the surface temperature of the oil tank. Under the comprehensive cooperation of these two functions, the heat in the tank will flow to the outer surface, thus realizing the net output of radiant energy from the outer surface, that is, providing refrigeration effect to the oil and gas in the tank.
2. Experimental subjects
2.1 Radiation refrigeration coating
Diesel oil is used as an absorption liquid to absorb foreign oil and gas in the tower, which is composed of a horizontal tank and a vertical tank. The tower is 4-5 m high and about 2 m in diameter. Transverse radiation refrigeration coating is composed of organic resin, functional filler, auxiliary agent, water, etc. The main color is white, and various colors can be adjusted by filling pigments. The radiation refrigeration temperature-reducing coating can simultaneously realize the functions of minimum range absorption of the incident energy of sunlight and maximum range emission of self-heat through infrared window radiation, thus achieving the effect of zero energy consumption refrigeration.
2.2 Experimental oil and gas storage tanks
In September 2019, radiation refrigeration coating was sprayed on the pretreatment No.3 absorption tower in Ningbo Zhongjin Petrochemical Plant. The bottom of the absorption tower is used to absorb foreign oil and gas in the tower with diesel oil as absorption liquid. It is composed of a horizontal tank and a vertical tank. The tower is 4 – 5 m high and about 2 m in diameter. The horizontal tank has no other internals, but only liquid diesel oil, which is used to continuously absorb the oil and gas in the vertical tank. Its purpose is to continuously absorb the oil and gas drained from large storage tanks in CICC Petrochemical Plant and reduce the oil and gas loss and environmental pollution.
The transmission pipeline, vacuum pump and other equipment are connected to the bottom of the absorption tower. Gasoline gas is imported from the lower air inlet of the vertical tank and exported from the upper air outlet. The temperature of the imported gas is 50-60℃. After liquefying, the gasoline gas is absorbed by diesel oil in the horizontal tank. After accumulated for a certain period of time, the horizontal tank discharges the diesel oil full of oil and gas, and discharges it into fresh diesel oil again, continuously absorbing oil and gas and continuously circulating. The circulation period is set to 12 h or 24 h. Liquefaction of gaseous oil is an exothermic process, and the increase of temperature will lead to the decrease of gas-liquid conversion rate. At the same time, too high a temperature will also lead to excessive pressure in the tank, which will lead to potential safety hazards. Therefore, it is particularly important to reduce the hot spots of high temperature in tanks and eliminate potential safety hazards. The temperature of the tank is affected by the internal heat of the working medium, heat generated by the working medium and external radiation heat gain. After spraying radiation refrigeration coating on the surface of the tank, the external radiation heat gain will be effectively reduced and the internal working environment will be improved.
3. Temperature test experiment
3.1 Construction method
The construction process includes cleaning, surface grinding, closed bottom spraying, primer spraying, topcoat spraying and topcoat spraying. After each spraying process, it needs to be naturally dried for 1 h before proceeding to the next process. The situation before and after the construction of the storage tank is shown in Figure 3.
3.2 Experimental scheme
3.2.1 Test place
Zhenhai District, Ningbo City, Zhejiang Province (29 57 N, 121 43 E).
3.2.2 Test time is from September to October 2019.
3.2.3 Experimental methods
WZY-1 temperature self-recording instrument was used to record the temperature changes of the tank wall surfaces of two absorption towers with and without radiation cooling coating, and the cooling effect was compared and analyzed. The temperature recorder continuously collects and records the temperature for 24 hours, with a sampling period of 1 min/time. The data was recorded from September 26th to October 7th and was tested in two stages. In the first stage, the measuring points were all located in the oil-gas mixing zone from September 26th to October 29th, and the measuring points were respectively placed at the left, middle and right positions of the same height of the horizontal tank. The second stage is the testing of different working medium areas from October 1 to 6. The measuring points are respectively placed in the upper (gas tank wall), middle (oil-gas mixing area tank wall), lower (liquid tank wall) of the horizontal tank and the middle of the vertical tank. The schematic diagram of the measuring points is shown in Figure 4. The temperature difference under different climatic conditions is compared and analyzed. The temperature difference is obtained by subtracting the temperature of the coating tank with radiation refrigeration from the temperature of the coating tank.
3.3 Testing of the same working medium area
At the stage, the measuring points of the first horizontal tank are located at the same height, which is all oil-gas mixing areas. From September 26 to September 27, the absorption liquid of the two pretreatment absorption towers was changed once every 12 hours. On 28th and 29th, the absorption liquid of two pretreatment absorption towers was changed once every 24 hours, and the results are shown in fig. 5. Change the absorption liquid twice a day: Under cloudy weather, the maximum temperature difference of the horizontal tank in the left, middle and right daytime reaches 11.6℃, 10.3℃ and 10.7℃ respectively; Under sunny weather conditions, the maximum temperature difference of horizontal tank in the left, middle and right daytime reaches 10.1℃, 8.5℃ and 8.7℃ respectively. Under the condition of changing the absorption liquid once a day: in cloudy weather, the maximum temperature difference in the left, middle and right daytime of the horizontal tank reaches 9.8℃, 6.9℃ and 8.7℃ respectively; Under sunny weather conditions, the maximum temperature difference of horizontal tank in the left, middle and right daytime reaches 9.7℃, 6.5℃ and 8.7℃, respectively. The results show that, no matter what operating conditions, the temperature at the measuring point of the absorption tower bottom with RADI-COOL radiation refrigeration coating for most of 24h is lower than that of the non-construction tank, and the temperature difference between the two sides of the horizontal tank is larger than that in the middle part. In different positions of the oil-gas mixing area, the temperature drop amplitude on both sides of the horizontal tank is 8. 7℃ greater than that in the middle of the horizontal tank. During the day, radiation refrigeration coatings can reduce the peak temperature of tanks under different meteorological conditions such as sunny days and cloudy days.
3.4 Test of different working medium areas
Phase II measuring points are arranged in the gas zone, oil-gas mixing zone, liquid zone and the middle of the storage tank of the horizontal tank. From Oct. 1 to Oct. 3, the bottoms of the two absorption towers operated in the same condition, and the absorption liquid was changed once every 24 hours. October 1 is a typhoon day, and the temperature difference between the upper and middle parts of the horizontal tank is small, while the temperature difference in the lower liquid area still reaches 4-6℃, and the temperature difference in the middle part of the vertical tank reaches 5.3℃. On October 2nd, it was cloudy. The maximum daytime temperature difference in the upper, middle and lower parts of the horizontal tank reached 13.7℃, 12.4℃ and 9.1℃, respectively, and the temperature difference in the middle part of the vertical tank reached 7.8℃. On October 3rd, it was sunny. The maximum daytime temperature difference in the upper, middle and lower parts of the horizontal tank reached 15. 6℃, 12.8℃ and 11. 1℃ respectively, and the temperature difference in the middle part of the vertical tank reached 9. 4℃. Compared with the daytime and nighttime data on sunny days, the daytime temperature difference can reach 15. 0℃, and the nighttime temperature difference is 0-5℃. The temperature reduction effect is more obvious during the daytime high-temperature period, which plays a very good role in “peak elimination”.
Compared with the data of October 1st and October 3rd, the cooling effect on sunny days is more prominent. Therefore, radiation refrigeration coating will effectively solve the problem of high temperature of storage tanks during sunny days. Comparing the temperature difference in different weather in the whole period, not only the absolute temperature of the tower bottom of the spray paint absorption tower is lower than that of the tower bottom without absorption tower, but also the variation range of temperature rise is relatively small when the air temperature rises, which is conducive to reducing respiratory loss.
1) There are some problems in oil and gas storage tanks at high temperatures, such as increased internal pressure and increased respiratory loss, etc. The use of coating for cooling has the advantages of low cost, remarkable effect and convenient modification, and the radiation refrigeration coating has the refrigeration effect that ordinary heat-insulating white paint does not have.
2) Radiation cooling coating can effectively and reliably provide passive cooling for storage tanks; After spraying radiation refrigeration coating on the surface of the oil and gas storage tank, the temperature of the tank wall during the day is lower than that of the unimplemented tank wall.15.0℃ is beneficial to reduce the heat in the tank.
3) After applying the radiation refrigeration coating, not only the absolute temperature is lower than that of the tower bottom without absorption, but also the change range of temperature rise is relatively small when the air temperature rises, which is beneficial to reduce the respiratory loss.