Utilization of Radiation Refrigeration Coatings in Storage Tanks for Oil and Gas

Oil and gas storage tanks experience continuous heat accumulation at elevated temperatures, leading to a rapid increase in surface and internal temperatures. To address the issue of increased internal pressure due to high temperatures, frequent vent valve openings are necessary, exacerbating product loss and causing air pollution. Exceeding the critical internal pressure may even result in an explosion. Generally, the storage temperature for light boiling point oil in tanks should not exceed 45°C, and the temperature difference between the inside and outside of liquefied natural gas (LNG) tanks can reach up to 200°C. Additionally, the significant temperature fluctuations between day and night in summer contribute to the "breathing phenomenon" of oil tanks, increasing the concentration of combustible oil and gas near the storage tank farm, wasting resources, and posing potential safety hazards.

In recent years, both domestic and international scholars have proposed the use of cold water spraying to reduce tank temperatures. However, this method has drawbacks, such as rust and dirt accumulation on the tank's outer wall, leading to mold, rust, and a shortened equipment service life. Moreover, dirt darkens the tank wall, intensifying sunlight absorption and causing localized temperature increases. These practices fail to address the fundamental issues of oil and gas loss and potential safety hazards, and they also result in significant water and electricity resource waste.

Among various related technologies, coating cooling stands out due to its low cost, remarkable effectiveness, and easy adaptability, showing promise in both building and industrial energy-saving applications.

Radiation refrigeration coating represents a novel coating type based on passive radiation refrigeration technology. It exhibits high infrared emission capability within the atmospheric transmission window band (8-13 μm) and efficiently transfers heat to cold sources in outer space. Simultaneously, it possesses high reflective ability to solar radiation energy, minimizing solar radiation heat absorption and enabling 24-hour zero-energy radiation refrigeration. The requirements for temperature reduction and emission reduction in the petrochemical industry align well with the metamaterial technology of radiation refrigeration. This passive radiation refrigeration technology offers significant daytime temperature reduction, effectively mitigating the risk of the working medium in the tank exceeding standard temperatures and enhancing the safety and reliability of equipment operation. Additionally, it helps suppress the "respiration" of oil and gas caused by day-to-night temperature differences, reducing material losses and alleviating environmental protection discharge pressures for enterprises. This technology holds substantial economic and social significance.

 
1. Mechanism of Surface Temperature Influence

The temperature of the tank's outer wall significantly impacts the state of the working medium inside the tank. According to Chen Qisheng's modeling, heat from the external environment permeates into the tank wall, leading to the evaporation loss of oil and gas within the tank. Simultaneously, heat infiltration results in an increase in the internal pressure of the tank, posing a potential safety hazard. The thermal resistance of the tank wall can be determined by the formula:
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 established, the heat flow into the tank is directly proportional to the temperature difference between the inside and outside of the tank (δ t). Therefore, finding a convenient and efficient method to reduce the temperature of the outer wall of the storage tank is crucial for decreasing the temperature of the working medium in the tank and minimizing the evaporation loss of oil and gas.
 
Typically, epoxy zinc-rich paint is applied to the surface of oil and gas storage tanks, serving to prevent corrosion and enhance tank wall reflection. In comparison to epoxy zinc-rich paint, radiation refrigeration paint exhibits higher solar reflectivity. In the visible light band, where solar energy is primarily concentrated, the reflectivity of the radiation refrigeration coating is nearly 95%. Consequently, when compared to common epoxy zinc-rich paint, the radiation refrigeration coating significantly reduces solar heat gain and alters the heat flow direction of the tank.

Theoretically, for refrigeration to occur, the solar reflectivity of the coating must exceed 88% when the solar radiation intensity is 900-1,000 W/m² at noon. Based on 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%, enabling refrigeration functionality without energy consumption.

The cooling effect of radiation cooling paint distinguishes it from ordinary heat-insulating white paint. When solar radiation acts on the surface of ordinary heat-reflective white paint, most of the energy is absorbed, heating the tank and creating a "heat source." Radiation refrigeration coating, with a solar reflectivity of 93%, prevents the surface of the storage tank from absorbing solar radiation. Additionally, the high emissivity of the infrared window continuously transfers heat to outer space, reducing the surface temperature of the oil tank. Through the combined action of these two functions, heat in the tank flows to the outer surface, achieving a net output of radiant energy from the outer surface and providing a refrigeration effect to the oil and gas in the tank.
 
2. Experimental Subjects

(1) Radiation Refrigeration Coating
Diesel oil is employed as an absorption liquid for capturing foreign oil and gas within a tower, comprising a horizontal tank and a vertical tank. The tower stands at a height of 4-5 m with a diameter of about 2 m. The transverse radiation refrigeration coating is crafted from organic resin, functional fillers, auxiliary agents, water, etc. The primary color is white, with customizable variations using pigments. The radiation refrigeration temperature-reducing coating achieves the dual functions of minimal-range absorption of incident sunlight energy and maximum-range emission of self-heat through infrared window radiation. This results in the effect of zero-energy consumption refrigeration.

(2) Experimental Oil and Gas Storage Tanks
In September 2019, radiation refrigeration coating was applied to the No.3 pretreatment absorption tower at the Ningbo Zhongjin Petrochemical Plant. The bottom of the absorption tower, using diesel oil as the absorption liquid, serves to absorb foreign oil and gas. The tower consists of a horizontal tank and a vertical tank, both approximately 4-5 m high and about 2 m in diameter. The horizontal tank contains only liquid diesel oil and is employed for continuously absorbing oil and gas from the vertical tank. The primary objective is to absorb oil and gas continuously discharged from large storage tanks in the CICC Petrochemical Plant, thereby minimizing oil and gas loss and reducing environmental pollution. Equipment such as transmission pipelines and vacuum pumps is connected to the bottom of the absorption tower. Gasoline gas is introduced through the lower air inlet of the vertical tank and exits from the upper air outlet. The imported gas temperature is maintained at 50-60°C. After liquefying, the gasoline gas is absorbed by diesel oil in the horizontal tank. After a specific accumulation period, the horizontal tank discharges the diesel oil containing oil and gas, subsequently replenishing it with fresh diesel oil to maintain a continuous absorption and circulation process. The circulation period is set to either 12 or 24 hours. Liquefaction of gaseous oil is an exothermic process, and excessive temperature can reduce the gas-liquid conversion rate, potentially leading to elevated pressure and safety hazards in the tank. Therefore, reducing high-temperature hot spots and mitigating potential safety risks are critical. The tank's temperature is influenced by internal heat from the working medium, heat generated by the working medium, and external radiation heat gain. The application of radiation refrigeration coating on the tank's surface effectively reduces external radiation heat gain, enhancing the internal working environment.
 
3. Temperature Test Experiment

(1) Construction Method
The construction process involves cleaning, surface grinding, closed bottom spraying, primer spraying, topcoat spraying, and topcoat spraying. Each spraying process requires natural drying for 1 hour before proceeding to the next step. The situation before and after the construction of the storage tank is illustrated in Figure 3.
(2) Experimental Scheme
Test Location
Zhenhai District, Ningbo City, Zhejiang Province (29°57'N, 121°43'E).
Test Period: September to October 2019.
Experimental Methods
The WZY-1 temperature self-recording instrument was employed to record temperature changes on the tank wall surfaces of two absorption towers, one with radiation cooling coating and one without. The cooling effects were compared and analyzed. The temperature recorder continuously collected and recorded temperature data over 24 hours, with a sampling period of 1 minute per reading. The data collection spanned from September 26th to October 7th, conducted in two stages. In the first stage, measuring points were positioned in the oil-gas mixing zone from September 26th to October 29th, with points at the left, middle, and right positions of the horizontal tank at the same height. The second stage, from October 1st to 6th, focused on different working medium areas. Measuring points were at the upper (gas tank wall), middle (oil-gas mixing area tank wall), and lower (liquid tank wall) of the horizontal tank, as well as the middle of the vertical tank. The schematic diagram of the measuring points is depicted in Figure 4. Temperature differences under varying weather conditions were compared and analyzed, calculated by subtracting the temperature of the coating tank with radiation refrigeration from the temperature of the tank without coating.

(3) Testing of the Same Working Medium Area
In this stage, measuring points for the first horizontal tank were at the same height in the oil-gas mixing area. From September 26th to 27th, the absorption liquid in both pretreatment absorption towers was changed every 12 hours. On the 28th and 29th, the absorption liquid was changed every 24 hours. Results are shown in Figure 5. Changing the absorption liquid twice a day under cloudy conditions resulted in maximum temperature differences on the left, middle, and right sides of the horizontal tank during the daytime, reaching 11.6°C, 10.3°C, and 10.7°C, respectively. Under sunny conditions, the maximum temperature differences in the left, middle, and right sides of the horizontal tank during the daytime were 10.1°C, 8.5°C, and 8.7°C, respectively. Changing the absorption liquid once a day under cloudy conditions resulted in maximum temperature differences in the left, middle, and right sides of the horizontal tank during the daytime, reaching 9.8°C, 6.9°C, and 8.7°C, respectively. Under sunny conditions, the maximum temperature differences in the left, middle, and right sides of the horizontal tank during the daytime were 9.7°C, 6.5°C, and 8.7°C, respectively. The results indicated that, under various operating conditions, the temperature at the measuring point of the absorption tower bottom with RADI-COOL radiation refrigeration coating was generally lower than that of the non-coated tank for the majority of the 24 hours. The temperature difference between the two sides of the horizontal tank was larger than that in the middle part of the tank. In different positions of the oil-gas mixing area, the temperature drop amplitude on both sides of the horizontal tank was 8.7°C greater than that in the middle of the tank. During the day, radiation refrigeration coatings demonstrated the ability to reduce peak tank temperatures under different meteorological conditions, such as sunny and cloudy days.

(4) Testing of Different Working Medium Areas
In the second phase, measuring points were arranged in the gas zone, oil-gas mixing zone, liquid zone, and the middle of the storage tank in the horizontal tank. From October 1st to October 3rd, the bottoms of the two absorption towers operated under the same conditions, changing the absorption liquid every 24 hours. On October 1st, a typhoon day, the temperature difference between the upper and middle parts of the horizontal tank was small, while the temperature difference in the lower liquid area still reached 4-6°C, and the temperature difference in the middle part of the vertical tank reached 5.3°C. On October 2nd, with cloudy weather, the maximum daytime temperature differences in the upper, middle, and lower parts of the horizontal tank reached 13.7°C, 12.4°C, and 9.1°C, respectively, and the temperature difference in the middle part of the vertical tank reached 7.8°C. On October 3rd, with sunny weather, the maximum daytime temperature differences in the upper, middle, and lower parts of the horizontal tank reached 15.6°C, 12.8°C, and 11.1°C, respectively, and the temperature difference in the middle part of the vertical tank reached 9.4°C. Comparing the data on sunny days during daytime and nighttime, the temperature difference during the daytime could reach 15.0°C, while the nighttime temperature difference was 0-5°C. The cooling effect was more evident during the high-temperature daytime period, proving effective in "peak elimination." Comparing the temperature differences under different weather conditions throughout the entire period, the tower bottom temperature of the radiation refrigeration coating-sprayed absorption tower was not only lower than that of the non-coated tower, but also exhibited a relatively small variation range when the air temperature rose, contributing to reduced respiratory loss.
 
Conclusion

Elevated temperatures pose challenges for oil and gas storage tanks, leading to issues like increased internal pressure and heightened respiratory loss. Coating solutions for cooling, particularly the radiation refrigeration coating, present advantages such as cost-effectiveness, remarkable efficacy, and easy adaptability, offering a cooling effect that conventional heat-insulating white paint lacks.

The application of radiation cooling coating proves to be a reliable and effective means of providing passive cooling for storage tanks. Following the application of radiation refrigeration coating on the surface of oil and gas storage tanks, the tank wall temperature during the day is lower than that of untreated tank walls, with a notable reduction of up to 15.0°C, contributing to the alleviation of heat within the tank.

The implementation of radiation refrigeration coating not only results in lower absolute temperatures compared to absorption towers without coating but also ensures a relatively modest range of temperature increase in response to rising air temperatures. This characteristic is advantageous in minimizing respiratory losses.