Oil and its products are a mixture of various hydrocarbons, among which the light components are highly volatile. In the process of oil extraction, refining, storage, transportation, sales and application, it is inevitable that some of the lighter liquid components will vaporize and be discharged into the atmosphere, causing oil loss and atmospheric pollution, which is very harmful.
According to the Fourth International Petroleum Conference in 1995, in the UK, the amount of oil loss from the well site to the refining and processing to the finished product sales accounts for about 3% of the total crude oil production. In 1980, the test results of 11 major oil fields in my country showed that from the wellhead to the mine crude oil depot, the oil loss in the mine accounted for about 2% of the oil production, of which the evaporation loss in the well station depot accounted for about 32% of the total consumption; the loss from the refinery production equipment to the loading and transportation process was 0.5%; plus the evaporation loss in the operation and storage process of the oil company, the total loss in my country was about 3%. Even in Japan, where oil is in short supply, the evaporation loss rate of its oil products is far more than 2%. If the total loss rate is estimated at 3%, the amount of oil products lost to the atmosphere worldwide each year is about 108 tons, which is almost equivalent to China's annual crude oil production. Most oil terminals, refineries, oil depots and gas stations are located in or around densely populated cities. The vapor emitted into the atmosphere contain benzene and organic reactive compounds (ROCs). Benzene can cause cancer in humans, and organic reactive compounds and nitrogen oxides will undergo a series of photochemical reactions under the action of ultraviolet rays, generating ozone (accounting for 85% of the reactants), acetyl nitrate monooxygenate (PAN accounts for about 10% of the reactants), and highly active free radicals (RO2, HO2, RCO Secondary pollutants such as aldehydes (formaldehyde, acetaldehyde, propionaldehyde), ketones and organic acids; SO in the atmosphere will also generate sulfate aerosols (i.e. photochemical aerosols). This mixture of primary and secondary pollutants is photochemical smog.
The consequences of photochemical smog are very serious. Toxic substances strongly irritate people's eyes and throats, cause headaches and worsen the condition of patients with respiratory diseases. In severe cases, it can cause death. Therefore, how to effectively reduce the evaporation loss of oil products has always been an important topic for oil storage and transportation professionals to study and solve.
1. Causes of evaporation loss of oil products
Oil loss can be divided into three forms: leakage, oil mixing and evaporation loss. The first two are mainly caused by poor management, improper equipment maintenance and operation, etc., which can be avoided by strengthening management in all aspects. Free. Evaporation loss is a natural loss and the largest type of oil loss, accounting for about 70% to 80% of the total oil storage and transportation loss. The internal cause is the fraction composition of the oil. The lighter the fraction, the lower the initial distillation point, the greater the vapor pressure, the more severe the evaporation, and the greater the evaporation loss. The external factors that promote the evaporation loss of oil products are mainly the temperature, the size of the space above the oil tank, the size of the oil tank, and breathing. Therefore, the main products prone to evaporation loss are solvent gasoline, aviation gasoline, motor gasoline, and crude oil. The evaporation loss of kerosene and diesel is relatively small, and the evaporation loss of lubricating oil is very small and can be ignored.
1. Temperature
The higher the storage temperature of the oil, the more severe the evaporation of the oil. For example, in southern my country, a steel tank with a volume of 10,000m: can store gasoline in summer, and the daily evaporation loss of gasoline can reach 500~1000kg.
2. Sealing degree of oil tank
The sealing degree of the steel welding of oil tank accessories and the top of the oil tank will cause evaporation loss of oil products. A 5000m3 oil tank, due to the loose hole cover, causes natural ventilation, and the monthly loss of gasoline can reach 53x103kg, and the loss of crude oil is 28x103kg.
3. vapor space above the oil tank
The larger the space above the oil in the oil tank, the greater the evaporation loss. For example, when storing the same gasoline at the same temperature and sealing conditions, the evaporation loss when the oil volume is 20% of the tank volume is 8 times greater than when the oil volume is 95% of the tank volume.
4. Natural ventilation loss
Natural ventilation loss is due to the fact that there are holes on the top of the tank or there is a height difference between the two holes. Because the density of the mixed gas is greater than the density of air, the mixed gas in the tank is discharged from the lower hole into the atmosphere, and the outside air flows into the tank from the higher hole. This loss caused by natural convection of gas due to the difference in hole position and gas density is called natural ventilation loss. Natural ventilation loss often occurs when the tank top and tank body are corroded and perforated or the welds have sand holes, the glass of the fire protection system foam chamber is damaged, the valve disc of the breathing valve is not tightly covered, the hydraulic valve is not filled with oil or the oil seal is insufficient, and the oil measuring hole and the light transmission hole are not covered. According to the investigation, a 5000m3 oil tank in a certain oil depot stored gasoline due to severe ventilation, and it was measured that 53t of gasoline was lost, with a loss rate of about 1.5%.
Since the causes of natural ventilation loss are both equipment problems and management problems, as long as management is strengthened and equipment is repaired in time, natural ventilation loss can be avoided.
5. "Small Breathing" Loss
When the oil in the tank is stored statically without receiving and sending operations, as the external temperature and pressure change in a day, the gas space temperature in the tank, the evaporation rate of the oil, the oil gas depth and the vapor pressure also change accordingly. The oil loss caused by the process of discharging oil vapor and inhaling air is called "small breathing" loss, which is usually also called static storage loss of oil tanks. The main reason for the "small breathing" loss of oil tanks is the change in atmospheric temperature. When the temperature rises from dawn with the increase of sunlight, and reaches the maximum value at about 4 to 5 pm, the temperature of the oil in the tank also changes, causing the evaporation rate to accelerate and the evaporation pressure to increase. When the pressure exceeds the positive pressure rating of the breathing valve, the vapor mixture in the tank is discharged into the atmosphere through the breathing valve. This is the "exhalation" process of the oil tank. After 5 pm, when the sunlight gradually weakens, the temperature of the oil in the tank also decreases, the evaporation rate gradually slows down, and the pressure in the tank also decreases. When the pressure in the tank is lower than the negative pressure rating specified by the breathing valve, the outside air is sucked into the tank through the open vacuum valve. This is the "inhalation" process of the oil tank. Therefore, the "exhalation" and "inhalation" processes of the oil tank change regularly and cyclically every day. Generally speaking, the duration of "exhalation" every day is longer than that of "inhalation". Relevant data show that the "small breathing" loss of a 5000m3 vault tank can reach 350kg in one day and night. A 1000m³ above-ground vaulted metal tank at a southern oil depot stores gasoline for a year, and the "small breathing" loss reaches 11.7t, which is large enough to affect its economic benefits.
